More thorough rewrite of "if"/"then"/"else"

Further feedback indicates the language was still confusing.
Rewrote it to be more explicit about how the keywords interact
with each other and with both validation and annotation
collection.

In particular, an "if" on its own collects annotations but
never affects validation, annotations are only collected from
"them" or "else" if their use is indicated by the validation
outcome of an adjacent "if", and there is no default behavior
for any of the keywords.

Technically, "then" could be assigned an empty schema default
without causing problems, but that felt weirdly unbalanced.

If "if" has a default of true/empty schema, then "then" on
its own would be active, which is counter-intuitive.  If "else"
had a default it would need to be false rather than true, and
saying it has no default seemed simpler, and matches how "not"
is handled.  "not" is the only other keyword that would otherwise
need a false schema as a default value.

Author: handrews

jsonschema-validation.xml

@@ -746,10 +746,23 @@
                     branch of an "allOf" MUST NOT have an impact on a "then"
                     or "else" in another branch.
                 </t>
+                <t>
+                    There is no default behavior for any of these keywords
+                    when they are not present.  In particular, they MUST NOT
+                    be treated as if present with an empty schema, and when
+                    "if" is not present, both "then" and "else" MUST be
+                    entirely ignored.
+                </t>
                 <section title="if">
                     <t>
                         This keyword's value MUST be a valid JSON Schema.
                     </t>
+                    <t>
+                        This validation outcome of this keyword's subschema
+                        has no direct effect on the overall validation
+                        result.  Rather, it controls which of the "then"
+                        or "else" keywords are evaluated.
+                    </t>
                     <t>
                         Instances that successfully validate against this
                         keyword's subschema MUST also be valid against
@@ -763,48 +776,46 @@
                         present.
                     </t>
                     <t>
-                        Validation of the instance against this keyword
-                        on its own always succeeds, regardless of the
-                        validation outcome of the instance against its subschema.
+                        If <xref target="annotations">annotations</xref>
+                        are being collected, they are collected from this
+                        keyword's subschema in the usual way, including when
+                        the keyword is present without either "then" or "else".
                     </t>
                 </section>
                 <section title="then">
                     <t>
                         This keyword's value MUST be a valid JSON Schema.
                     </t>
                     <t>
-                        When present alongside of "if", the instance
-                        successfully validates against this keyword if
-                        it validates against both the "if"'s subschema
-                        and this keyword's subschema.
+                        When "if" is present, and the instance successfully
+                        validates against its subschema, then valiation
+                        succeeds against this keyword if the instance also
+                        successfully validates against this keyword's subschema.
                     </t>
                     <t>
-                        When "if" is absent, or the instance fails to
-                        validate against its subschema, validation against
-                        this keyword always succeeds, regardless of the
-                        validation outcome of the instance against its subschema.
-                        Implementations SHOULD avoid attempting to validate against
-                        the subschema in these cases.
+                        This keyword has no effect when "if" is absent, or
+                        when the instance fails to validate against its
+                        subschema.  Implementations MUST NOT evaluate
+                        the instance against this keyword, for either validation
+                        or annotation collection purposes, in such cases.
                     </t>
                 </section>
                 <section title="else">
                     <t>
                         This keyword's value MUST be a valid JSON Schema.
                     </t>
                     <t>
-                        When present alongside of "if", the instance
-                        successfully validates against this keyword if
-                        it fails to validate against the "if"'s
-                        subschema, and successfully validates against
-                        this keyword's subschema.
+                        When "if" is present, and the instance fails to
+                        validate against its subschema, then valiation
+                        succeeds against this keyword if the instance
+                        successfully validates against this keyword's subschema.
                     </t>
                     <t>
-                        When "if" is absent, or the instance successfully
-                        validates against its subschema, validation against
-                        this keyword always succeeds, regardless of the
-                        validation outcome of the instance against its subschema.
-                        Implementations SHOULD avoid attempting to validate against
-                        the subschema in these cases.
+                        This keyword has no effect when "if" is absent, or
+                        when the instance successfully validates against its
+                        subschema.  Implementations MUST NOT evaluate
+                        the instance against this keyword, for either validation
+                        or annotation collection purposes, in such cases.
                     </t>
                 </section>
             </section>