Can't distinguish between single SpecializeDef and mapSubject/mapPredicate, so disallow.

Peter Amstutz · Peter Amstutz · commit f814cdbced81 · 2016-07-07T13:16:50.000-04:00
diff --git a/schema_salad/metaschema/metaschema.yml b/schema_salad/metaschema/metaschema.yml
@@ -265,7 +265,6 @@ $graph:
 
     - name: specialize
       type:
-        - SpecializeDef?
         - SpecializeDef[]?
       doc: |
         Only applies if `extends` is declared.  Apply type specialization using the
