Add "anchorPointer" for in-instance contexts

This will need a bit more work after "anchor" is merged in,
as the improved wording about what the default context is
is all in that other PR.  But the key points are well enough
developed for review.

In the interst of getting the main points out for review, I
referenced the long-expired Relative JSON Pointer I-D.  Maybe
that's OK since this is not even a working group document yet,
or we can talk about how to either move the relevant parts in,
or re-submit that I-D ourselves as well.

This goes along with "hrefPointers", which adjusts template
variable resolution in the same way that this keyword adjusts
context location.

Author: handrews

jsonschema-hyperschema.xml

@@ -9,6 +9,7 @@
 <!ENTITY rfc5988 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.5988.xml">
 <!ENTITY rfc6570 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.6570.xml">
 <!ENTITY rfc7231 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.7231.xml">
+<!ENTITY I-D.luff-relative-json-pointer SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.draft-luff-relative-json-pointer-00.xml">
 <!ENTITY I-D.reschke-http-jfv SYSTEM "https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.draft-reschke-http-jfv-06.xml">
 ]>
 <?rfc toc="yes"?>
@@ -793,6 +794,91 @@ GET /foo/
                 </t>
             </section>
 
+            <section title="anchorPointer" anchor="anchorPointer">
+                <t>
+                    This property changes the point within the instance that is considered
+                    to be the context resource of the link.  The value of the property is a
+                    <xref target="I-D.luff-relative-json-pointer">Relative JSON Pointer</xref>,
+                    starting from the default context (the part of the instance that validated
+                    against the schema containing the LDO).
+                </t>
+                <t>
+                    While an alternate context with a known URI is best set with the
+                    "anchor" keyword, the lack of a fragment identifier
+                    syntax for application/json means that it is usually not possible to
+                    change the context within a JSON instance.
+                </t>
+                <t>
+                    Even in "+json" media types that define JSON Pointer as a fragment identifier
+                    syntax, if the default context is nested within an array, it is not possible to
+                    obtain the index of the default context's position in that array in order
+                    to construct a pointer to another property in that same nested JSON object.
+                </t>
+
+                <figure>
+                    <preamble>
+                        For example, given this hyper-schema:
+                    </preamble>
+                    <artwork>
+<![CDATA[{
+    "type": "object",
+    "properties": {
+        "theThing": {
+            "type": "object",
+            "properties": {
+                "name": {"type": "string"}
+            }
+        },
+        "examples": {
+            "title": "The collection of example for the thing",
+            "type": "array",
+            "items": {
+                "type": "object",
+                "properties": {
+                    "id": {"type": "integer"}
+                },
+                "links": [{
+                    "rel": "item",
+                    "href": "/examples/{id}",
+                    "anchorPointer": "1"
+                }]
+            }
+        }
+    }
+}]]>
+                    </artwork>
+                    <postamble>
+                        The "item" relation indicates that each array entry can be used to construct
+                        a link from the collection embedded in this instance to a resource for that
+                        specific item.  The default context of this link is the individual array
+                        element.  But the context of an "item" relation should be the collection,
+                        which is the entire "examples" array.  The Relative JSON Pointer of "1"
+                        accomplishes this as follows:
+                    </postamble>
+                </figure>
+                <figure>
+                    <preamble>
+                        Given this instance of media type application/json:
+                    </preamble>
+                    <artwork>
+<![CDATA[{
+    "theThing": {"name": "An actual thing"},
+    "examples": [{"id": 1234}, {"id": 5678}]
+}]]>
+                    </artwork>
+                    <postamble>
+                        The default context for the link to "/examples/1234" is the first
+                        array entry, and for the link to "/examples/5678" it is the second array
+                        entry.  But "anchorPointer" moves this up one instance level (the "1"
+                        Relative JSON Pointer) to be the array containing each entry.
+                    </postamble>
+                </figure>
+                <t>
+                    If both "anchor" and "anchorPointer" are defined, and do not resolve to
+                    the same link context, the resulting behavior is undefined.
+                </t>
+            </section>
+
             <section title="title">
                 <t>
                     This property defines a title for the link.
@@ -1190,6 +1276,7 @@ GET /foo/
             &rfc3986;
             <!--&rfc4287;-->
             &rfc6570;
+            &I-D.luff-relative-json-pointer;
             &I-D.reschke-http-jfv;
             <reference anchor="json-schema">
                 <front>