Default behaviors; annotation-dependent results

Describe how default behaviors work in more detail, covering
all three major keyword categories.

Allow keywords to depend on the presence or absence of adjacent
keywords (this is already true with "additionalItems" and "items').

Allow keywords to depend on the annotation results of subschemas
and adjacent keywords.  This is the fundamental mechanism behind
the forthcoming "unevaluatedProperties" and "unevaluatedItems"
keywords.

The behavior of "additionalProperties" and "additionalItems"
can be described with this mechanism (along with the presence
or absence mechanism) as well, which will serve as a proof
of concept of the approach in a subsequent commit.

Author: handrews

jsonschema-core.xml

@@ -126,8 +126,9 @@
             <section title="Keyword Behaviors">
                 <t>
                     JSON Schema keywords fall into several general behavior categories.
-                    Assertions validate that an instance satisfies constraints.  Annotations
-                    attach information that applications may use in any way they see fit.
+                    Assertions validate that an instance satisfies constraints, producing
+                    a boolean result.  Annotations attach information that applications
+                    may use in any way they see fit.
                     Applicators apply subschemas to parts of the instance and combine
                     their results.
                 </t>
@@ -145,6 +146,38 @@
                     subschemas have been evaluated, although in some circumstance evaluation
                     may be short-circuited due to assertion results.
                 </t>
+                <t>
+                    Keyword behavior MAY be defined in terms of the annotation results
+                    of <xref target="root">subschemas</xref> and/or adjacent keywords.
+                    Such keywords MUST NOT result in a circular dependency.
+                    Keywords MAY modify their behavior based on the presence or absence
+                    of another keyword in the same
+                    <xref target="schema-document">schema object</xref>.
+                </t>
+                <t>
+                    A missing keyword MUST NOT produce a false assertion result, MUST
+                    NOT produce annotation results, and MUST NOT cause any other schema
+                    to be evaluated as part of its own behavioral definition.
+                    However, given that missing keywords do not contribute annotations,
+                    the lack of annotation results may indirectly change the behavior
+                    of other keywords.
+                </t>
+                <t>
+                    In some cases, the missing keyword assertion behavior of a keyword is
+                    identical to that produced by a certain value, and keyword definitions
+                    SHOULD note such values where known.  However, even if the value which
+                    produces the default behavior would produce annotation results if
+                    present, the default behavior still MUST NOT result in annotations.
+                </t>
+                <t>
+                    Because annotation collection can add significant cost in terms of both
+                    computation and memory, implementations MAY opt out of this feature.
+                    Keywords known to an implementation to have assertion or applicator behavior
+                    that depend on annotation results MUST then be treated as errors, unless
+                    an alternate implementation producing the same behavior is available.
+                    Keywords of this sort SHOULD describe reasonable alternate approaches
+                    when appropriate.
+                </t>
                 <t>
                     Extension keywords SHOULD stay within these categories, keeping in mind
                     that annotations in particular are extremely flexible.  Complex behavior
@@ -190,9 +223,7 @@
                     </t>
                     <t>
                         An instance can only fail an assertion that is present in the schema.
-                        In some cases, this no-op behavior is identical to a keyword that exists with
-                        certain values, and keyword definitions SHOULD note such values where known.
-                        These default behaviors MUST NOT result in assertion failures.
+
                     </t>
                     <section title="Assertions and Instance Primitive Types">
                         <t>