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Versions: (draft-kwatsen-netmod-artwork-folding) 00 01 02

NETMOD Working Group                                           K. Watsen
Internet-Draft                                           Watsen Networks
Intended status: Best Current Practice                         A. Farrel
Expires: October 8, 2019                              Old Dog Consulting
                                                                   Q. Wu
                                                     Huawei Technologies
                                                           April 6, 2019


     Handling Long Lines in Inclusions in Internet-Drafts and RFCs
                  draft-ietf-netmod-artwork-folding-02

Abstract

   This document defines a simple and yet time-proven strategy for
   handling long lines in inclusions in internet drafts and RFCs using a
   backslash ('\') character to indicate where line-folding has
   occurred.  The strategy works on any text-based content, but is
   primarily intended for a structured sequence of lines rather than for
   two-dimensional imagery.  The approach produces consistent results,
   regardless of the content, that is both self-documenting and enables
   automated reconstitution of the original content.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 8, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Applicability Statement . . . . . . . . . . . . . . . . . . .   3
   3.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   4.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Automated Folding of Long Lines in Text Content . . . . .   4
     4.2.  Automated Reconstitution of the Original Text Content . .   4
   5.  Limitations . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Not Recommended for Graphical Artwork . . . . . . . . . .   5
     5.2.  Doesn't Work as Well as Format-Specific Options . . . . .   5
   6.  Two Folding Strategies  . . . . . . . . . . . . . . . . . . .   5
     6.1.  Comparison  . . . . . . . . . . . . . . . . . . . . . . .   6
     6.2.  Recommendation  . . . . . . . . . . . . . . . . . . . . .   6
   7.  The Single Backslash Strategy ('\') . . . . . . . . . . . . .   6
     7.1.  Folded Structure  . . . . . . . . . . . . . . . . . . . .   6
       7.1.1.  Header  . . . . . . . . . . . . . . . . . . . . . . .   6
       7.1.2.  Body  . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.2.  Algorithm . . . . . . . . . . . . . . . . . . . . . . . .   7
       7.2.1.  Folding . . . . . . . . . . . . . . . . . . . . . . .   7
       7.2.2.  Unfolding . . . . . . . . . . . . . . . . . . . . . .   8
   8.  The Double Backslash Strategy ('\\')  . . . . . . . . . . . .   9
     8.1.  Folded Structure  . . . . . . . . . . . . . . . . . . . .   9
       8.1.1.  Header  . . . . . . . . . . . . . . . . . . . . . . .   9
       8.1.2.  Body  . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.2.  Algorithm . . . . . . . . . . . . . . . . . . . . . . . .  10
       8.2.1.  Folding . . . . . . . . . . . . . . . . . . . . . . .  10
       8.2.2.  Unfolding . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.1.  Example Showing Boundary Conditions . . . . . . . . . . .  12
       9.1.1.  Using '\' . . . . . . . . . . . . . . . . . . . . . .  12
       9.1.2.  Using '\\'  . . . . . . . . . . . . . . . . . . . . .  12
     9.2.  Example Showing Multiple Wraps of a Single Line . . . . .  13
       9.2.1.  Using '\' . . . . . . . . . . . . . . . . . . . . . .  13
       9.2.2.  Using '\\'  . . . . . . . . . . . . . . . . . . . . .  13
     9.3.  Example Showing Smart Folding . . . . . . . . . . . . . .  13
       9.3.1.  Using '\' . . . . . . . . . . . . . . . . . . . . . .  13
       9.3.2.  Using '\\'  . . . . . . . . . . . . . . . . . . . . .  14
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  15
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16



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     12.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     12.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Appendix A.  POSIX Shell Script . . . . . . . . . . . . . . . . .  17
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   [RFC7994] sets out the requirements for plain-text RFCs and states
   that each line of an RFC (and hence of an Internet-Draft) must be
   limited to 72 characters followed by the character sequence that
   denotes an end-of-line (EOL).

   Internet-Drafts and RFCs often include example text or code
   fragments.  Many times the example text or code exceeds the 72
   character line-length limit.  The `xml2rfc` utility does not attempt
   to wrap the content of such inclusions, simply issuing a warning
   whenever lines exceed 69 characters.  According to the RFC Editor,
   there is currently no convention in place for how to handle long
   lines in such inclusions, other than advising authors to clearly
   indicate what manipulation has occurred.

   This document introduces a simple and yet time-proven strategy for
   handling long lines using a backslash ('\') character to indicate
   where line-folding has occurred.  The strategy works on any text
   based inclusion, but is primarily intended for a structured sequence
   of lines, such as would be referenced by the <sourcecode> element
   defined in Section 2.48 of [RFC7991], rather than for two-dimensional
   imagery, such as would be referenced by the <artwork> element defined
   in Section 2.5 of [RFC7991].  The approach produces consistent
   results, regardless of the content, that is both self-documenting and
   enables automated reconstitution of the original content.

   Note that text files are represented as lines having their first
   character in column 1, and a line length of N where the last
   character is in the Nth column and is immediately followed by an end
   of line character sequence.

2.  Applicability Statement

   The format and algorithm defined in this document may be used in any
   context, whether for IETF documents or in other situations where
   structured folding is desired.

   Within the IETF, this work primarily targets the xml2rfc v3
   <sourcecode> element (Section 2.48 of [RFC7991]) and the xml2rfc v2
   <artwork> element (Section 2.5 of [RFC7749]) that, for lack of a
   better option, is currently used for both source code and artwork.



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   This work may be also be used for the xml2rfc v3 <artwork> element
   (Section 2.5 of [RFC7991]) but, as described in Section 5.1, it is
   generally not recommended.

3.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

4.  Goals

4.1.  Automated Folding of Long Lines in Text Content

   Automated folding of long lines is needed in order to support draft
   compilations that entail a) validation of source input files (e.g.,
   XML, JSON, ABNF, ASN.1) and/or b) dynamic generation of output, using
   a tool that doesn't observe line lengths, that is stitched into the
   final document to be submitted.

   Generally, in order for tooling to be able to process input files,
   the files must be in their original/natural state, which may entail
   them having some long lines.  Thus, these source files need to be
   modified before inclusion in the document in order to satisfy the
   line length limits.  This modification SHOULD be automated to reduce
   effort and errors resulting from manual processing.

   Similarly, dynamically generated output (e.g., tree diagrams) must
   also be modified, if necessary, in order for the resulting document
   to satisfy the line length limits.  When needed, this effort again
   SHOULD be automated to reduce effort and errors resulting from manual
   processing.

4.2.  Automated Reconstitution of the Original Text Content

   Automated reconstitution of the original content is needed to support
   validation of text-based inclusions extracted from documents.  YANG
   [RFC7950] modules are already extracted from Internet-Drafts and
   validated as part of the draft-submission process.  Additionally,
   there has been some discussion regarding needing to also validate
   instance examples (i.e., XML/JSON documents) contained within
   Internet-Drafts ([yang-doctors-thread]).  Thus, it SHOULD be possible
   to mechanically reconstitute the original text content in order to
   utilize such tooling.





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5.  Limitations

5.1.  Not Recommended for Graphical Artwork

   While the solution presented in this document will work on any kind
   of text-based content, it is most useful on content that represents
   source code (XML, JSON, etc.) or, more generally, on content that has
   not been laid out in two dimensions (e.g., diagrams).

   Fundamentally, the issue is whether the text content remains readable
   once folded.  Text content that is unpredictable is especially
   susceptible to looking bad when folded; falling into this category
   are most UML diagrams, YANG tree diagrams, and ASCII art in general.

   It is NOT RECOMMENDED to use the solution presented in this document
   on graphical artwork.

5.2.  Doesn't Work as Well as Format-Specific Options

   The solution presented in this document works generically for all
   text-based content, as it only views content as plain text.  However,
   various formats sometimes have built-in mechanisms that are better
   suited to prevent long lines.

   For instance, both the `pyang` and `yanglint` utilities have the
   command line option "--tree-line-length" that can be used to indicate
   a desired maximum line length for when generating tree diagrams
   [RFC8340].

   In another example, some source formats (e.g., YANG [RFC7950]) allow
   any quoted string to be broken up into substrings separated by a
   concatenation character (e.g., '+'), any of which can be on a
   different line.

   In yet another example, some languages allow factoring blocks of code
   into call outs, such as functions.  Using such call outs is
   especially helpful when in some deeply-nested code, as they typically
   reset the indentation back to the first column.

   It is RECOMMENDED that authors do as much as possible within the
   selected format to avoid long lines.

6.  Two Folding Strategies

   This document defines two nearly identical strategies for folding
   text-based content.





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      The Single Backslash Strategy ('\'):  Uses a backslash ('\')
         character at the end of the line where folding occurs, and
         assumes that the continuation begins at the first non-
         whitespace character on the following line.

      The Double Backslash Strategy ('\\'):  Uses a backslash ('\')
         character at the end of the line where folding occurs, and
         assumes that the continuation begins after a second backslash
         ('\') character on the following line.

6.1.  Comparison

   The first strategy produces more readable output, however it is
   significantly more likely to encounter unfoldable input (e.g., there
   is exists a line anywhere in the input ending with a backslash
   character, or there exists a long line containing only space and
   backslash characters) and, for long lines that can be folded,
   automation implementations are likely to encounter scenarios that
   will produce errors without special care.

   The second strategy produces less readable output, but is unlikely to
   encounter unfoldable input, there are no long lines that cannot be
   folded, and no special care is required for when folding a long line.

6.2.  Recommendation

   It is RECOMMENDED for implementations to first attempt to fold
   content using the single backslash strategy and, only in the unlikely
   event that it cannot fold the input or the folding logic is unable to
   cope with a contingency occurring on the desired folding column, then
   fallback to the double backslash strategy.

7.  The Single Backslash Strategy ('\')

7.1.  Folded Structure

   Text content that has been folded as specified by this strategy MUST
   adhere to the following structure.

7.1.1.  Header

   The header is two lines long.

   The first line is the following 45-character string that MAY be
   surrounded by any number of printable characters.  This first line
   cannot itself be folded.

   NOTE: '\' line wrapping per BCP XX (RFC XXXX)



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   [Note to RFC Editor: Please replace XX and XXXX with the numbers
   assigned to this document and delete this note.  Please make this
   change in multiple places in this document.]

   The second line is a blank line.  This line provides visual
   separation for readability.

7.1.2.  Body

   The character encoding is the same as described in Section 2 of
   [RFC7994], except that, per [RFC7991], tab characters are prohibited.

   Lines that have a backslash ('\') occurring as the last character in
   a line are considered "folded".

   Really long lines may be folded multiple times.

7.2.  Algorithm

   This section describes the process for folding and unfolding long
   lines when they are encountered in a single instance of text content.
   It is assumed that another process inserts/extracts the individual
   text content instances to/from an Internet-Draft or RFC.  For
   example, the `xiax` utility [xiax] does this.

7.2.1.  Folding

   Folding is assumed to be automated although authors may perform the
   folding steps manually.

   Determine the desired maximum line length from input to the automated
   line-wrapping process, such as from a command line parameter.  If no
   value is explicitly specified, the value "69" SHOULD be used.

   Ensure that the desired maximum line length is not less than the
   minimum header, which is 46 characters.  If the desired maximum line
   length is less than this minimum, exit (this text-based content
   cannot be folded).

   Scan the text content for horizontal tab characters.  If any
   horizontal tab characters appear, either resolve them to space
   characters or exit, forcing the input provider to convert them to
   space characters themselves first.

   Scan the text content to see if any line exceeds the desired maximum.
   If no line exceeds the desired maximum, exit (this text content does
   not need to be folded).




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   Scan the text content to ensure no existing lines already end with a
   backslash ('\') character, as this would lead to an ambiguous result.
   If such a line is found, exit (this text content cannot be folded).

   If this text content needs to and can be folded, insert the header
   described in Section 7.1.1, ensuring that any additional printable
   characters surrounding the header does not result in a line exceeding
   the desired maximum..

   For each line in the text content, from top-to-bottom, if the line
   exceeds the desired maximum, then fold the line by:

   1.  Determine where the fold will occur.  This location MUST be
       before or at the desired maximum column, and MUST NOT precede a
       space (' ') character.

   2.  At the location where the fold is to occur, insert a backslash
       ('\') character followed by the end of line character sequence.

   3.  On the following line, insert any number of space (' ')
       characters.

   The result of the previous operation is that the next line starts
   with an arbitrary number of space (' ') characters, followed by the
   character that was previously occupying the position where the fold
   occurred.

   Continue in this manner until reaching the end of the text content.
   Note that this algorithm naturally addresses the case where the
   remainder of a folded line is still longer than the desired maximum,
   and hence needs to be folded again, ad infinitum.

   The process described in this section is illustrated by the
   "fold_it_1()" function in Appendix A.

7.2.2.  Unfolding

   All unfolding is assumed to be automated, although a reader will
   mentally perform the act of unfolding the text to understand the true
   nature of the original text content.

   Scan the beginning of the text content for the header described in
   Section 7.1.1.  If the header is not present, starting on the first
   line of the text content, exit (this text contents does not need to
   be unfolded).

   Remove the 2-line header from the text content.




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   For each line in the text content, from top-to-bottom, if the line
   has a backslash ('\') character immediately followed by the end of
   line character sequence, then the line can be unfolded.  Remove the
   backslash ('\') character, the end of line character sequence, and
   any leading space (' ') characters, which will bring up the next
   line.  Then continue to scan each line in the text content starting
   with the current line (in case it was multiply folded).

   Continue in this manner until reaching the end of the text content.

   The process described in this section is illustrated by the
   "unfold_it_1()" function in Appendix A.

8.  The Double Backslash Strategy ('\\')

8.1.  Folded Structure

   Text content that has been folded as specified by this strategy MUST
   adhere to the following structure.

8.1.1.  Header

   The header is two lines long.

   The first line is the following 46-character string that MAY be
   surrounded by any number of printable characters.  This first line
   cannot itself be folded.

   NOTE: '\\' line wrapping per BCP XX (RFC XXXX)

   [Note to RFC Editor: Please replace XX and XXXX with the numbers
   assigned to this document and delete this note.  Please make this
   change in multiple places in this document.]

   The second line is a blank line.  This line provides visual
   separation for readability.

8.1.2.  Body

   The character encoding is the same as described in Section 2 of
   [RFC7994], except that, per [RFC7991], tab characters are prohibited.

   Lines that have a backslash ('\') occurring as the last character in
   a line immediately followed by the end of line character sequence,
   when the subsequent line starts with a backslash ('\') as the first
   non-space (' ') character, are considered "folded".

   Really long lines may be folded multiple times.



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8.2.  Algorithm

   This section describes the process for folding and unfolding long
   lines when they are encountered in a single instance of text content.
   It is assumed that another process inserts/extracts the individual
   text content instances to/from an Internet-Draft or RFC.  For
   example, the `xiax` utility [xiax] does this.

8.2.1.  Folding

   Folding is assumed to be automated, although authors may perform the
   folding steps manually.

   Determine the desired maximum line length from input to the automated
   line-wrapping process, such as from a command line parameter.  If no
   value is explicitly specified, the value "69" SHOULD be used.

   Ensure that the desired maximum line length is not less than the
   minimum header, which is 45 characters.  If the desired maximum line
   length is less than this minimum, exit (this text-based content
   cannot be folded).

   Scan the text content for horizontal tab characters.  If any
   horizontal tab characters appear, either resolve them to space
   characters or exit, forcing the input provider to convert them to
   space characters themselves first.

   Scan the text content to see if any line exceeds the desired maximum.
   If no line exceeds the desired maximum, exit (this text content does
   not need to be folded).

   Scan the text content to ensure no existing lines already end with a
   backslash ('\') character while the subsequent line starts with a
   backslash ('\') character as the first non-space (' ') character, as
   this would lead to an ambiguous result.  If such a line is found,
   exit (this text content cannot be folded).

   If this text content needs to and can be folded, insert the header
   described in Section 8.1.1, ensuring that any additional printable
   characters surrounding the header does not result in a line exceeding
   the desired maximum..

   For each line in the text content, from top-to-bottom, if the line
   exceeds the desired maximum, then fold the line by:

   1.  Determine where the fold will occur.  This location MUST be
       before or at the desired maximum column.




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   2.  At the location where the fold is to occur, insert a first
       backslash ('\') character followed by the end of line character
       sequence.

   3.  On the following line, insert any number of space (' ')
       characters followed by a second backslash ('\') character.

   The result of the previous operation is that the next line starts
   with an arbitrary number of space (' ') characters, followed by a
   backslash ('\') character, immediately followed by the character that
   was previously occupying the position where the fold occurred.

   Continue in this manner until reaching the end of the text content.
   Note that this algorithm naturally addresses the case where the
   remainder of a folded line is still longer than the desired maximum,
   and hence needs to be folded again, ad infinitum.

   The process described in this section is illustrated by the
   "fold_it_2()" function in Appendix A.

8.2.2.  Unfolding

   All unfolding is assumed to be automated although a reader will
   mentally perform the act of unfolding the text to understand the true
   nature of the original text content.

   Scan the beginning of the text content for the header described in
   Section 8.1.1.  If the header is not present, starting on the first
   line of the text content, exit (this text content does not need to be
   unfolded).

   Remove the 2-line header from the text content.

   For each line in the text content, from top-to-bottom, if the line
   has a backslash ('\') character immediately followed by the end of
   line character sequence, and if the next line has a backslash ('\')
   character as the first non-space (' ') character, then the lines can
   be unfolded.  Remove the first backslash ('\') character, the end of
   line character sequence, any leading space (' ') characters, and the
   second backslash ('\') character, which will bring up the next line.
   Then continue to scan each line in the text content starting with the
   current line (in case it was multiply folded).

   Continue in this manner until reaching the end of the text content.

   The process described in this section is illustrated by the
   "unfold_it_2()" function in Appendix A.




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9.  Examples

   The following self-documenting examples illustrate folded text-based
   content.

   The source text content cannot be presented here, as it would again
   be folded.  Alas, only the results can be provided.

9.1.  Example Showing Boundary Conditions

   This example illustrates boundary condition.  The input contains
   seven lines, each line one character longer than the previous line.
   Numbers for counting purposes.  The default desired maximum column
   value "69" is used.

9.1.1.  Using '\'

   =========== NOTE: '\' line wrapping per BCP XX (RFC XXXX) ===========

   123456789012345678901234567890123456789012345678901234567890123456
   1234567890123456789012345678901234567890123456789012345678901234567
   12345678901234567890123456789012345678901234567890123456789012345678
   123456789012345678901234567890123456789012345678901234567890123456789
   12345678901234567890123456789012345678901234567890123456789012345678\
   90
   12345678901234567890123456789012345678901234567890123456789012345678\
   901
   12345678901234567890123456789012345678901234567890123456789012345678\
   9012

9.1.2.  Using '\\'

   ========== NOTE: '\\' line wrapping per BCP XX (RFC XXXX) ===========

   123456789012345678901234567890123456789012345678901234567890123456
   1234567890123456789012345678901234567890123456789012345678901234567
   12345678901234567890123456789012345678901234567890123456789012345678
   123456789012345678901234567890123456789012345678901234567890123456789
   12345678901234567890123456789012345678901234567890123456789012345678\
   \90
   12345678901234567890123456789012345678901234567890123456789012345678\
   \901
   12345678901234567890123456789012345678901234567890123456789012345678\
   \9012







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9.2.  Example Showing Multiple Wraps of a Single Line

   This example illustrates what happens when very long line needs to be
   folded multiple times.  The input contains one line containing 280
   characters.  Numbers for counting purposes.  The default desired
   maximum column value "69" is used.

9.2.1.  Using '\'

   =========== NOTE: '\' line wrapping per BCP XX (RFC XXXX) ===========

   12345678901234567890123456789012345678901234567890123456789012345678\
   90123456789012345678901234567890123456789012345678901234567890123456\
   78901234567890123456789012345678901234567890123456789012345678901234\
   56789012345678901234567890123456789012345678901234567890123456789012\
   34567890

9.2.2.  Using '\\'

   ========== NOTE: '\\' line wrapping per BCP XX (RFC XXXX) ===========

   12345678901234567890123456789012345678901234567890123456789012345678\
   \9012345678901234567890123456789012345678901234567890123456789012345\
   \6789012345678901234567890123456789012345678901234567890123456789012\
   \3456789012345678901234567890123456789012345678901234567890123456789\
   \01234567890

9.3.  Example Showing Smart Folding

   This example illustrates how readability can be improved via "smart"
   folding, whereby folding occurs at format-specific locations and
   format-specific indentations are used.

   The text content was manually folded, since the script in the
   appendix does not implement smart folding.

   Note that the header is surrounded by different printable characters
   then shown in the script-generated examples.

9.3.1.  Using '\'











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   [NOTE: '\' line wrapping per BCP XX (RFC XXXX)]

   <yang-library
       xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library"
       xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">

     <module-set>
       <name>config-modules</name>
       <module>
         <name>ietf-interfaces</name>
         <revision>2018-02-20</revision>
         <namespace>\
           urn:ietf:params:xml:ns:yang:ietf-interfaces\
         </namespace>
       </module>
       ...
     </module-set>
     ...
   </yang-library>

   Below is the equivalent to the above, but it was folded using the
   script in the appendix.

   =========== NOTE: '\' line wrapping per BCP XX (RFC XXXX) ===========

   <yang-library
       xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library"
       xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">

     <module-set>
       <name>config-modules</name>
       <module>
         <name>ietf-interfaces</name>
         <revision>2018-02-20</revision>
         <namespace>urn:ietf:params:xml:ns:yang:ietf-interfaces</namesp\
   ace>
       </module>
       ...
     </module-set>
     ...
   </yang-library>

9.3.2.  Using '\\'








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   [NOTE: '\\' line wrapping per BCP XX (RFC XXXX)]

   <yang-library
       xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library"
       xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">

     <module-set>
       <name>config-modules</name>
       <module>
         <name>ietf-interfaces</name>
         <revision>2018-02-20</revision>
         <namespace>\
           \urn:ietf:params:xml:ns:yang:ietf-interfaces\
         \</namespace>
       </module>
       ...
     </module-set>
     ...
   </yang-library>

   Below is the equivalent to the above, but it was folded using the
   script in the appendix.

   ========== NOTE: '\\' line wrapping per BCP XX (RFC XXXX) ===========

   <yang-library
       xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library"
       xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">

     <module-set>
       <name>config-modules</name>
       <module>
         <name>ietf-interfaces</name>
         <revision>2018-02-20</revision>
         <namespace>urn:ietf:params:xml:ns:yang:ietf-interfaces</namesp\
   \ace>
       </module>
       ...
     </module-set>
     ...
   </yang-library>

10.  Security Considerations

   This BCP has no Security Considerations.






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11.  IANA Considerations

   This BCP has no IANA Considerations.

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

12.2.  Informative References

   [RFC7749]  Reschke, J., "The "xml2rfc" Version 2 Vocabulary",
              RFC 7749, DOI 10.17487/RFC7749, February 2016,
              <https://www.rfc-editor.org/info/rfc7749>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [RFC7991]  Hoffman, P., "The "xml2rfc" Version 3 Vocabulary",
              RFC 7991, DOI 10.17487/RFC7991, December 2016,
              <https://www.rfc-editor.org/info/rfc7991>.

   [RFC7994]  Flanagan, H., "Requirements for Plain-Text RFCs",
              RFC 7994, DOI 10.17487/RFC7994, December 2016,
              <https://www.rfc-editor.org/info/rfc7994>.

   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
              <https://www.rfc-editor.org/info/rfc8340>.

   [xiax]     "The `xiax` Python Package",
              <https://pypi.org/project/xiax/>.

   [yang-doctors-thread]
              "[yang-doctors] automating yang doctor reviews",
              <https://mailarchive.ietf.org/arch/msg/yang-doctors/
              DCfBqgfZPAD7afzeDFlQ1Xm2X3g>.





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Appendix A.  POSIX Shell Script

   This non-normative appendix section includes a shell script that can
   both fold and unfold text content.  Note that this script is applied
   only to single text content instances.

   #!/bin/bash --posix   # must be `bash` (not `sh`)

   print_usage() {
     echo
     echo "Folds the text file, only if needed, at the specified"
     echo "column, according to BCP XX."
     echo
     echo "Usage: $0 [-s <strategy>] [-c <col>] [-r] -i <infile>"
     echo "                                              -o <outfile>"
     echo
     echo "  -s: strategy to use, '1' or '2' (default: try 1, else 2)"
     echo "  -c: column to fold on (default: 69)"
     echo "  -r: reverses the operation"
     echo "  -i: the input filename"
     echo "  -o: the output filename"
     echo "  -d: show debug messages"
     echo "  -h: show this message"
     echo
     echo "Exit status code: zero on success, non-zero otherwise."
     echo
   }

   # global vars, do not edit
   strategy=0 # auto
   debug=0
   reversed=0
   infile=""
   outfile=""
   maxcol=69  # default, may be overridden by param
   hdr_txt_1="NOTE: '\\' line wrapping per BCP XX (RFC XXXX)"
   hdr_txt_2="NOTE: '\\\\' line wrapping per BCP XX (RFC XXXX)"
   equal_chars="=============================================="
   space_chars="                                              "

   fold_it_1() {
     # ensure input file doesn't contain the fold-sequence already
     pcregrep -M  "\\\\\n" $infile >> /dev/null 2>&1
     if [ $? -eq 0 ]; then
       echo
       echo "Error1: infile $infile has a line ending with a '\\'"
       echo "character. This file cannot be folded."
       echo



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       return 1
     fi

     # stash some vars
     testcol=`expr "$maxcol" + 1`
     foldcol=`expr "$maxcol" - 1` # for the inserted '\' char

     # ensure input file doesn't contain whitespace on the fold column
     grep "^.\{$foldcol\} " $infile >> /dev/null 2>&1
     if [ $? -eq 0 ]; then
       echo
       echo "Error: infile has a space character occuring after the"
       echo "folding column. This file cannot be folded."
       echo
       return 1
     fi

     # center header text
     length=`expr ${#hdr_txt_1} + 2`
     left_sp=`expr \( "$maxcol" - "$length" \) / 2`
     right_sp=`expr "$maxcol" - "$length" - "$left_sp"`
     header=`printf "%.*s %s %.*s" "$left_sp" "$equal_chars"\
                      "$hdr_txt_1" "$right_sp" "$equal_chars"`

     # generate outfile
     echo "$header" > $outfile
     echo "" >> $outfile
     gsed "/.\{$testcol\}/s/\(.\{$foldcol\}\)/\1\\\\\n/g"\
             < $infile >> $outfile

     return 0
   }

   fold_it_2() {
     # ensure input file doesn't contain the fold-sequence already
     pcregrep -M  "\\\\\n[\ ]*\\\\" $infile >> /dev/null 2>&1
     if [ $? -eq 0 ]; then
       echo
       echo "Error2: infile has a line ending with a '\\' character"
       echo "followed by a '\\' character as the first non-space"
       echo "character on the next line.  This file cannot be folded."
       echo
       return 1
     fi

     # center header text
     length=`expr ${#hdr_txt_2} + 2`
     left_sp=`expr \( "$maxcol" - "$length" \) / 2`



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     right_sp=`expr "$maxcol" - "$length" - "$left_sp"`
     header=`printf "%.*s %s %.*s" "$left_sp" "$equal_chars"\
                      "$hdr_txt_2" "$right_sp" "$equal_chars"`

     # fold using recursive passes ('g' used in fold_it_1 didn't work)
     if [ -z "$1" ]; then
       # init recursive env
       cp $infile /tmp/wip
     fi
     testcol=`expr "$maxcol" + 1`
     foldcol=`expr "$maxcol" - 1` # for the inserted '\' char
     gsed "/.\{$testcol\}/s/\(.\{$foldcol\}\)/\1\\\\\n\\\\/" < /tmp/wip\
           >> /tmp/wip2
     diff /tmp/wip /tmp/wip2 > /dev/null 2>&1
     if [ $? -eq 1 ]; then
       mv /tmp/wip2 /tmp/wip
       fold_it_2 "recursing"
     else
       echo "$header" > $outfile
       echo "" >> $outfile
       cat /tmp/wip2 >> $outfile
       rm /tmp/wip*
     fi
     return 0
   }

   fold_it() {
     # ensure input file doesn't contain a TAB
     grep $'\t' $infile >> /dev/null 2>&1
     if [ $? -eq 0 ]; then
       echo
       echo "Error: infile contains a TAB character, which is not"
       echo "allowed."
       echo
       return 1
     fi

     # check if file needs folding
     testcol=`expr "$maxcol" + 1`
     grep ".\{$testcol\}" $infile >> /dev/null 2>&1
     if [ $? -ne 0 ]; then
       if [[ $debug -eq 1 ]]; then
         echo "nothing to do"
       fi
       cp $infile $outfile
       return -1
     fi




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     if [[ $strategy -eq 1 ]]; then
       fold_it_1
       return $?
     fi
     if [[ $strategy -eq 2 ]]; then
       fold_it_2
       return $?
     fi
     fold_it_1
     if [ $? -ne 0 ]; then
       fold_it_2
       return $?
     fi
     return 0
   }

   unfold_it_1() {
     # output all but the first two lines (the header) to wip file
     awk "NR>2" $infile > /tmp/wip

     # unfold wip file
     gsed ":x; /.*\\\\$/N; s/\\\\\n[ ]*//; tx" /tmp/wip > $outfile

     # clean up and return
     rm /tmp/wip
     return 0
   }

   unfold_it_2() {
     # output all but the first two lines (the header) to wip file
     awk "NR>2" $infile > /tmp/wip

     # unfold wip file
     gsed ":x; /.*\\\\$/N; s/\\\\\n[ ]*\\\\//; tx" /tmp/wip > $outfile

     # clean up and return
     rm /tmp/wip
     return 0
   }

   unfold_it() {
     # check if file needs unfolding
     line=`head -n 1 $infile`
     result=`echo $line | fgrep "$hdr_txt_1"`
     if [ $? -eq 0 ]; then
       unfold_it_1
       return $?
     fi



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     result=`echo $line | fgrep "$hdr_txt_2"`
     if [ $? -eq 0 ]; then
       unfold_it_2
       return $?
     fi
     if [[ $debug -eq 1 ]]; then
       echo "nothing to do"
     fi
     cp $infile $outfile
     return -1
   }

   process_input() {
     while [ "$1" != "" ]; do
       if [ "$1" == "-h" -o "$1" == "--help" ]; then
         print_usage
         exit 1
       fi
       if [ "$1" == "-d" ]; then
         debug=1
       fi
       if [ "$1" == "-s" ]; then
         strategy="$2"
         shift
       fi
       if [ "$1" == "-c" ]; then
         maxcol="$2"
         shift
       fi
       if [ "$1" == "-r" ]; then
         reversed=1
       fi
       if [ "$1" == "-i" ]; then
         infile="$2"
         shift
       fi
       if [ "$1" == "-o" ]; then
         outfile="$2"
         shift
       fi
       shift
     done

     if [ -z "$infile" ]; then
       echo
       echo "Error: infile parameter missing (use -h for help)"
       echo
       exit 1



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     fi

     if [ -z "$outfile" ]; then
       echo
       echo "Error: outfile parameter missing (use -h for help)"
       echo
       exit 1
     fi

     if [ ! -f "$infile" ]; then
       echo
       echo "Error: specified file \"$infile\" is does not exist."
       echo
       exit 1
     fi

     if [[ $strategy -eq 2 ]]; then
       min_supported=`expr ${#hdr_txt_2} + 8`
     else
       min_supported=`expr ${#hdr_txt_1} + 8`
     fi
     if [ $maxcol -lt $min_supported ]; then
       echo
       echo "Error: the folding column cannot be less than"
       echo "$min_supported."
       echo
       exit 1
     fi

     # this is only because the code otherwise runs out of equal_chars
     max_supported=`expr ${#equal_chars} + 1 + ${#hdr_txt_1} + 1\
          + ${#equal_chars}`
     if [ $maxcol -gt $max_supported ]; then
       echo
       echo "Error: the folding column cannot be more than"
       echo "$max_supported."
       echo
       exit 1
     fi
   }

   main() {
     if [ "$#" == "0" ]; then
        print_usage
        exit 1
     fi

     process_input $@



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     if [[ $reversed -eq 0 ]]; then
       fold_it
       code=$?
     else
       unfold_it
       code=$?
     fi
     exit $code
   }

   main "$@"

Acknowledgements

   The authors thank the following folks for their various contributions
   (sorted by first name): Benoit Claise, Gianmarco Bruno, Italo Busi,
   Joel Jaeggli, Jonathan Hansford, Lou Berger, Martin Bjorklund, and
   Rob Wilton.

   The authors additionally thank the RFC Editor for confirming that
   there is no set convention today for handling long lines in artwork/
   sourcecode inclusions.

Authors' Addresses

   Kent Watsen
   Watsen Networks

   EMail: kent+ietf@watsen.net


   Adrian Farrel
   Old Dog Consulting

   EMail: adrian@olddog.co.uk


   Qin Wu
   Huawei Technologies

   EMail: bill.wu@huawei.com










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