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Network Working Group                                           E. Wilde
Internet-Draft                                                       EMC
Intended status: Standards Track                        February 8, 2013
Expires: August 12, 2013


                 A Media Type for XML Patch Operations
                        draft-wilde-xml-patch-02

Abstract

   The XML Patch media type "application/xml-patch+xml" defines an XML
   document structure for expressing a sequence of patch operations that
   are applied to an XML document.  The XML Patch document format's
   foundations are defined in RFC 5261, this specification defines a
   document format and a media type registration, so that XML Patch
   documents can be labeled with a media type, for example in HTTP
   conversations.

Note to Readers

   This draft should be discussed on the apps-discuss mailing list [13].

   Online access to all versions and files is available at github [14].

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 http://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 August 12, 2013.

Copyright Notice

   Copyright (c) 2013 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



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   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   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.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  3
   3.  Patch Document Format  . . . . . . . . . . . . . . . . . . . .  4
   4.  Patch Examples . . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   6.  Implementation Hints . . . . . . . . . . . . . . . . . . . . .  6
     6.1.  Namespace Matching Rules . . . . . . . . . . . . . . . . .  6
     6.2.  Patching Namespaces  . . . . . . . . . . . . . . . . . . .  7
   7.  Change Log . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     7.1.  From -01 to -02  . . . . . . . . . . . . . . . . . . . . .  9
     7.2.  From -00 to -01  . . . . . . . . . . . . . . . . . . . . .  9
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     8.2.  Non-Normative References . . . . . . . . . . . . . . . . .  9
   Appendix A.  XSD from RFC 5261 . . . . . . . . . . . . . . . . . . 10
   Appendix B.  ABNF for RFC 5261 . . . . . . . . . . . . . . . . . . 12
   Appendix C.  Acknowledgements  . . . . . . . . . . . . . . . . . . 13
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13





















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1.  Introduction

   The Extensible Markup Language (XML) [1] is a common format for the
   exchange and storage of structured data.  HTTP PATCH [6] extends HTTP
   [7] with a method to perform partial modifications to resources.
   HTTP PATCH requires that patch documents are being sent along with
   the request, and it is therefore useful if there are standardized
   patch document formats (identified by media types) for popular media
   types.

   The XML Patch media type "application/xml-patch+xml" is an XML
   document structure for expressing a sequence of operations to apply
   to a target XML document, suitable for use with the HTTP PATCH
   method.  Servers can freely choose which patch formats they want to
   accept, and "application/xml-patch+xml" could be a simple default
   format that can be used unless a server decides to use a different
   (maybe more sophisticated) patch format for XML.

   The format for patch documents is based on the XML Patch Framework
   defined in RFC 5261 [2].  While RFC 5261 does define a concrete
   syntax as well as the media type "application/patch-ops-error+xml"
   for error documents, it only defines XML Schema (XSD) [8] types for
   patch operations, and thus the concrete document format and the media
   type for patch operations are defined in an XSD defined in this
   specification.


2.  IANA Considerations

   The Internet media type [3] for an XML Patch Document is application/
   xml-patch+xml.

      Type name: application

      Subtype name: xml-patch+xml

      Required parameters: none

      Optional parameters: Same as charset parameter for the media type
      "application/xml" as specified in RFC 3023 [1].

      Encoding considerations: Same as encoding considerations of media
      type "application/xml" as specified in RFC 3023 [1].

      Security considerations: This media type has all of the security
      considerations described in RFC 3023 [1] and RFC 5261 [2], plus
      those listed in Section 5.




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      Interoperability considerations: N/A

      Published specification: RFC XXXX

      Applications that use this media type: Applications that
      manipulate XML documents.

      Additional information:

         Magic number(s): N/A

         File extension(s): XML documents should use ".xml" as the file
         extension.

         Macintosh file type code(s): TEXT

      Person & email address to contact for further information: Erik
      Wilde <erik.wilde@emc.com>

      Intended usage: COMMON

      Restrictions on usage: none

      Author: Erik Wilde <erik.wilde@emc.com>

      Change controller: IETF


3.  Patch Document Format

   The XML patch document format is based on a simple schema that uses a
   "patch" element as the document element, and allows an arbitrary
   sequence of "add", "remove", and "replace" elements as the children
   of the document element.  These children follow the semantics defined
   in RFC 5261, which means that each element is treated as an
   individual patch operation, and the result of each patch operation is
   a patched XML document that is the target XML document for the next
   patch operation.

   The following example patch document uses the example from RFC 5261,
   and simply uses a "patch" element and a new XML namespace.  It shows
   the general structure of an XML patch document, as well as an example
   for each operation.








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<p:patch xmlns="urn:ietf:params:xml:ns:xxx" xmlns:y="urn:ietf:params:xml:ns:yyy"
    xmlns:p="urn:ietf:rfc:XXXX" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="urn:ietf:rfc:XXXX patch-document-00.xsd">
    <p:add sel="doc/elem[@a='foo']">
        <!-- This is a new child -->
        <child id="ert4773">
            <y:node/>
        </child>
    </p:add>
    <p:replace sel="doc/note/text()">Patched doc</p:replace>
    <p:remove sel="*/elem[@a='bar']/y:child" ws="both"/>
    <p:add sel="*/elem[@a='bar']" type="@b">new attr</p:add>
</p:patch>

   As this example demonstrates, both the document element "patch" and
   the patch operation elements are in the same XML namespace.  This is
   the result of RFC 5261 only defining types for the patch operation
   elements, which then can be reused in schemas to define concrete
   patch elements.

   RFC 5261 defines an XML Schema (XSD) [8] for the patch operation
   types, which is included for reference in Appendix A.  The normative
   version of this schema is the one given in RFC 5261.  The following
   schema for the XML Patch media type is based on the types defined in
   RFC 5261, which are imported as "rfc5261.xsd" in the following
   schema.  The schema defines a "patch" document element, and then
   allows an unlimited (and possibly empty) sequence of the "add",
   "remove", and "replace" operation elements, which are directly based
   on the respective types from the schema defined in RFC 5261.
<xs:schema targetNamespace="urn:ietf:rfc:XXXX"
    xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">
    <xs:import schemaLocation="rfc5261.xsd"/>
    <xs:element name="patch">
        <xs:complexType>
            <xs:choice minOccurs="0" maxOccurs="unbounded">
                <xs:element name="add" type="add"/>
                <xs:element name="remove" type="remove"/>
                <xs:element name="replace" type="replace"/>
            </xs:choice>
        </xs:complexType>
    </xs:element>
</xs:schema>


4.  Patch Examples

   Since the semantics of the XML patch operations are defined by RFC
   5261, please refer to the numerous examples in that specification for



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   concrete XML patch document examples.  Most importantly, the examples
   in RFC 5261 can be taken literally as examples for the XML Patch
   media type, as long as it is assumed that the XML namespace for the
   operation elements in these examples is the URI "urn:ietf:rfc:XXXX".


5.  Security Considerations

   ...


6.  Implementation Hints

   This section is informative.  It described some issues that might be
   interesting for implementors, but it might also be interesting for
   users of XML Patch that want to understand some of the differences
   between standard XPath 1.0 processing, and the processing model of
   RFC 5261.

6.1.  Namespace Matching Rules

   RFC 5261 defines standard rules for matching prefixed names in
   expressions: Any prefixes are interpreted according to the namespace
   bindings of the diff document (the document that the expression is
   applied against).  This means that each prefixed name can be
   interpreted in the context of the diff document.

   For unprefixed names in expressions, the rules depart from XPath 1.0
   [9].  XPath 1.0 defines that unprefixed names in expressions match
   namespace-less names (i.e., there is no "default namespace" for names
   used in XPath 1.0 expressions).  RFC 5261 requires, however, that
   unprefixed names in expressions must use the default namespace of the
   diff document (if there is one).  This means that it is not possible
   to simply take a selector from a patch document and evaluate it in
   the context of the diff document according to the rules of XPath 1.0,
   because this would interpret unprefixed names incorrectly.  As a
   consequence, it is not possible to simply take an XPath 1.0 processor
   and evaluate XMPL Patch selectors in the context of the diff
   document.

   As an extension of XPath 1.0's simple model, XPath 2.0 [10] specifies
   different processing rules for unprefixed names: They are matched
   against the URI of the "default element/type namespace", which is
   defined as part of an expression's static context.  In some XPath 2.0
   applications this can be set; XSLT 2.0 for example has the ability to
   define an "xpath-default-namespace", which then will be used to match
   unprefixed names in expressions.  Thus, by using an XPath 2.0
   implementation that allows to set this URI, and setting it to the



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   default namespace of the diff document (or leaving it undefined if
   there is no such default namespace), it is possible to use an out-of-
   the-box XPath 2.0 implementation for evaluating XML Patch selectors.

   Please keep in mind, however, that evaluating selectors is only one
   part of applying patches.  When it comes to applying the actual patch
   operations, neither XPath 1.0 nor XPath 2.0 are sufficient, because
   they are not preserving some of the information from the XML syntax
   (specifically: namespace declarations) that is required to correctly
   apply patch operations.  The following section described this issue
   in more detail.

   [[[ Currently, RFC 5261's section on namespace matching explains
   XPath 2.0's rules incorrectly
   <http://tools.ietf.org/html/rfc5261#section-4.2.2>.  An erratum has
   been filed
   <http://www.rfc-editor.org/errata_search.php?rfc=5261&eid=3477>
   which, upon verification, will be linked to from here. ]]]

6.2.  Patching Namespaces

   One of the issues when patching namespaces based on XPath is that
   XPath exposes namespaces different than the XML 1.0 [11] syntax for
   XML Namespaces [12].  In the XML syntax, a namespace is declared with
   an attribute using the reserved name or prefix "xmlns", and this
   results in this namespace being available recursively through the
   document tree.  In XPath, the namespace declaration is not exposed as
   an attribute (i.e., the attribute, although syntactically an XML
   attribute, is not accessible in XPath), but the namespace nodes are
   exposed recursively through the tree.

   RFC 5261 uses the terms "namespace declaration" and "namespace"
   almost interchangeably, but it is important to keep in mind that the
   namespace declaration is an XML syntax construct that is unavailable
   in XPath, while the namespace itself is a logical construct that is
   not visible in the XML syntax, but a result of a namespace
   declaration.  The intent of RFC 5261 is to patch namespaces as if
   namespace declarations were patched, and thus it only allows to patch
   namespace nodes on the element nodes where the namespace has been
   declared.

   Patching namespaces in XML Patch is supposed to "emulate" the effect
   of actually changing the namespace declaration (which is why a
   namespace can only be patched at the element where it has been
   declared).  Therefore, when patching a namespace, even though XPath's
   "namespace" axis is used, implementations have to make sure that not
   only the one selected namespace node is being patched, but that all
   namespaces nodes resulting from the namespace declaration of this



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   namespace are patched accordingly.

   This means that an implementation might have to descend into the
   tree, matching all namespace nodes with the selected prefix/URI pair
   recursively, until it encounters namespace declarations with the same
   prefix it is patching.  Determining this requires access to the diff
   document beyond XPath, because in XPath itself namespace declarations
   are not represented, and thus such a recursive algorithm wouldn't
   know when to stop.  Consider the following document:
   <x xmlns:a="tag:42">
       <y xmlns:a="tag:42"/>
   </x>

   If this document is patched with a selector of /x/namespace::a, then
   only the namespace node on element x should be patched, even though
   the namespace node on element y has the same prefix/URI combination
   than the one on element x.  Determining that the repeated namespace
   declaration was present at all on element y is impossible when using
   XPath alone, so implementations must have an alternative way to
   determine the difference between the document above, and this one:
   <x xmlns:a="tag:42">
       <y/>
   </x>

   In this second example, patching with a selector of /x/namespace::a
   should indeed change the namespace nodes on elements x and y, because
   they both have been derived from the same namespace declaration.

   The conclusion of these considerations is that for implementing XML
   Patch, access to the XML syntax (specifically: namespace
   declarations) is necessary.  As a result, implementations attempting
   to exclusively use the XPath model for implementing XML Patch will
   fail to correctly address certain edge cases (as the one shown
   above).

   [[[ Currently, RFC 5261's section on replacing namespaces mixes the
   terms "namespace declaration" and "namespace"
   <http://tools.ietf.org/html/rfc5261#section-4.4.3>.  An erratum has
   been filed <http://www.rfc-editor.org/errata_search.php?eid=3478>
   which, upon verification, will be linked to from here. ]]]


7.  Change Log

   Note to RFC Editor: Please remove this section before publication.






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7.1.  From -01 to -02

   o  Textual edits.

   o  Added section on "Implementation Hints" (Section 6).

7.2.  From -00 to -01

   o  Removed Mark Nottingham from author list.

   o  Changed media type name to application/xml-patch+xml (added suffix
      per draft-ietf-appsawg-media-type-suffix-regs)

   o  Added ABNF grammar derived from XSD (Appendix B)


8.  References

8.1.  Normative References

   [1]   Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types",
         RFC 3023, January 2001.

   [2]   Urpalainen, J., "An Extensible Markup Language (XML) Patch
         Operations Framework Utilizing XML Path Language (XPath)
         Selectors", RFC 5261, September 2008.

   [3]   Freed, N., Klensin, J., and T. Hansen, "Media Type
         Specifications and Registration Procedures", BCP 13, RFC 6838,
         January 2013.

   [4]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
         Extensions (MIME) Part One: Format of Internet Message Bodies",
         RFC 2045, November 1996.

   [5]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
         Extensions (MIME) Part Two: Media Types", RFC 2046,
         November 1996.

8.2.  Non-Normative References

   [6]   Dusseault, L. and J. Snell, "PATCH Method for HTTP", RFC 5789,
         March 2010.

   [7]   Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
         Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
         HTTP/1.1", RFC 2616, June 1999.




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   [8]   Thompson, H., Beech, D., Maloney, M., and N. Mendelsohn, "XML
         Schema Part 1: Structures Second Edition", World Wide Web
         Consortium Recommendation REC-xmlschema-1-20041028,
         October 2004,
         <http://www.w3.org/TR/2004/REC-xmlschema-1-20041028>.

   [9]   DeRose, S. and J. Clark, "XML Path Language (XPath) Version
         1.0", World Wide Web Consortium Recommendation REC-xpath-
         19991116, November 1999,
         <http://www.w3.org/TR/1999/REC-xpath-19991116>.

   [10]  Boag, S., Berglund, A., Kay, M., Simeon, J., Robie, J.,
         Chamberlin, D., and M. Fernandez, "XML Path Language (XPath)
         2.0 (Second Edition)", World Wide Web Consortium
         Recommendation REC-xpath20-20101214, December 2010,
         <http://www.w3.org/TR/2010/REC-xpath20-20101214>.

   [11]  Sperberg-McQueen, C., Yergeau, F., Paoli, J., Maler, E., and T.
         Bray, "Extensible Markup Language (XML) 1.0 (Fifth Edition)",
         World Wide Web Consortium Recommendation REC-xml-20081126,
         November 2008, <http://www.w3.org/TR/2008/REC-xml-20081126>.

   [12]  Hollander, D., Layman, A., Bray, T., Tobin, R., and H.
         Thompson, "Namespaces in XML 1.0 (Third Edition)", World Wide
         Web Consortium Recommendation REC-xml-names-20091208,
         December 2009,
         <http://www.w3.org/TR/2009/REC-xml-names-20091208>.

URIs

   [13]  <https://www.ietf.org/mailman/listinfo/apps-discuss>

   [14]  <https://github.com/dret/I-D/tree/master/xml-patch>


Appendix A.  XSD from RFC 5261

   For reference, this section contains a copy of the XML Schema (XSD)
   [8] defining the add, replace, and remove types in RFC 5261 [2].
   This section is informational only, and the definitive version of the
   schema is the one listed in RFC 5261.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE schema [
    <!ENTITY ncname "\i\c*">
    <!ENTITY qname  "(&ncname;:)?&ncname;">
    <!ENTITY aname  "@&qname;">
    <!ENTITY pos    "\[\d+\]">
    <!ENTITY attr   "\[&aname;='(.)*'\]|\[&aname;=&quot;(.)*&quot;\]">



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    <!ENTITY valueq "\[(&qname;|\.)=&quot;(.)*&quot;\]">
    <!ENTITY value  "\[(&qname;|\.)='(.)*'\]|&valueq;">
    <!ENTITY cond   "&attr;|&value;|&pos;">
    <!ENTITY step   "(&qname;|\*)(&cond;)*">
    <!ENTITY piq    "processing-instruction\((&quot;&ncname;&quot;)\)">
    <!ENTITY pi     "processing-instruction\(('&ncname;')?\)|&piq;">
    <!ENTITY id     "id\(('&ncname;')?\)|id\((&quot;&ncname;&quot;)?\)">
    <!ENTITY com    "comment\(\)">
    <!ENTITY text   "text\(\)">
    <!ENTITY nspa   "namespace::&ncname;">
    <!ENTITY cnodes "(&text;(&pos;)?)|(&com;(&pos;)?)|((&pi;)(&pos;)?)">
    <!ENTITY child  "&cnodes;|&step;">
    <!ENTITY last   "(&child;|&aname;|&nspa;)">
    ]>
<xsd:schema
    xmlns:xsd="http://www.w3.org/2001/XMLSchema"
    elementFormDefault="qualified">

    <xsd:simpleType name="xpath">
        <xsd:restriction base="xsd:string">
            <xsd:pattern
                value="(/)?((&id;)((/&step;)*(/&last;))?|(&step;/)*(&last;))"/>
        </xsd:restriction>
    </xsd:simpleType>

    <xsd:simpleType name="xpath-add">
        <xsd:restriction base="xsd:string">
            <xsd:pattern
                value="(/)?((&id;)((/&step;)*(/&child;))?|(&step;/)*(&child;))"/>
        </xsd:restriction>
    </xsd:simpleType>

    <xsd:simpleType name="pos">
        <xsd:restriction base="xsd:string">
            <xsd:enumeration value="before"/>
            <xsd:enumeration value="after"/>
            <xsd:enumeration value="prepend"/>
        </xsd:restriction>
    </xsd:simpleType>

    <xsd:simpleType name="type">
        <xsd:restriction base="xsd:string">
            <xsd:pattern value="&aname;|&nspa;"/>
        </xsd:restriction>
    </xsd:simpleType>

    <xsd:complexType name="add">
        <xsd:complexContent mixed="true">



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            <xsd:restriction base="xsd:anyType">
                <xsd:sequence>
                    <xsd:any processContents="lax" namespace="##any"
                        minOccurs="0" maxOccurs="unbounded"/>
                </xsd:sequence>
                <xsd:attribute name="sel" type="xpath-add"
                    use="required"/>
                <xsd:attribute name="pos" type="pos"/>
                <xsd:attribute name="type" type="type"/>
            </xsd:restriction>
        </xsd:complexContent>
    </xsd:complexType>

    <xsd:complexType name="replace">
        <xsd:complexContent mixed="true">
            <xsd:restriction base="xsd:anyType">
                <xsd:sequence>
                    <xsd:any processContents="lax" namespace="##any"
                        minOccurs="0" maxOccurs="1"/>
                </xsd:sequence>
                <xsd:attribute name="sel" type="xpath" use="required"/>
            </xsd:restriction>
        </xsd:complexContent>
    </xsd:complexType>

    <xsd:simpleType name="ws">
        <xsd:restriction base="xsd:string">
            <xsd:enumeration value="before"/>
            <xsd:enumeration value="after"/>
            <xsd:enumeration value="both"/>
        </xsd:restriction>
    </xsd:simpleType>

    <xsd:complexType name="remove">
        <xsd:attribute name="sel" type="xpath" use="required"/>
        <xsd:attribute name="ws" type="ws"/>
    </xsd:complexType>

</xsd:schema>


Appendix B.  ABNF for RFC 5261

   RFC 5261 [2] does not contain an ABNF grammar for the allowed subset
   of XPath expressions, but includes an XSD-based grammar in its type
   definition for operation types (which is shown in Appendix A).  In
   order to make implementation easier, this appendix contains an ABNF
   grammar that has been derived from the XSD expressions given in RFC



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   5261.  In the following grammar, "xpath" is the definition for the
   allowed XPath expressions for remove and replace operations, and
   "xpath-add" is the definition for the allowed XPath expressions for
   add operations.  The names of all grammar productions are the ones
   used in the XSD-based grammar of RFC 5261.
ncname     =  1*%x00-ffffffff
qname      =  [ ncname ":" ] ncname
aname      =  "@" qname
pos        =  "[" 1*DIGIT "]"
attr       =  ( "[" aname "='" 1*%x00-ffffffff "']" ) / ( "[" aname "=" DQUOTE 1*%x00-ffffffff DQUOTE "]" )
valueq     =  "[" ( qname / "." ) "=" DQUOTE 1*%x00-ffffffff DQUOTE "]"
value      =  ( "[" ( qname / "." ) "='" 1*%x00-ffffffff "']" ) / valueq
cond       =  attr / value / pos
step       =  ( qname / "*" ) 0*( cond )
piq        =  "processing-instruction(" [ DQUOTE ncname DQUOTE ] ")"
pi         =  ( "processing-instruction(" [ "'" ncname "'" ] ")" ) / piq
id         =  ( "id(" [ "'" ncname "'" ] ")" ) / ( "id(" [ DQUOTE ncname DQUOTE ] ")" )
com        =  "comment()"
text       =  "text()"
nspa       =  "namespace::" ncname
cnodes     =  ( text / com / pi ) [ pos ]
child      =  cnodes / step
last       =  child / aname / nspa
xpath      =  [ "/" ] ( ( id [ 0*( "/" step ) "/" last ] ) / ( 0*( step "/" ) last ) )
xpath-add  =  [ "/" ] ( ( id [ 0*( "/" step ) "/" child ] ) / ( 0*( step "/" ) child ) )


Appendix C.  Acknowledgements

   Thanks for comments and suggestions provided by Bas de Bakker.


Author's Address

   Erik Wilde
   EMC

   Email: erik.wilde@emc.com













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