[UR] Address review feedback

scripts/core/CONTRIB.rst

@@ -219,9 +219,10 @@ implement your experimental feature.
 Experimental features *must* not make any changes to the core YaML files and 
 *must* be described entirely in their own YaML file. Sometimes, however 
 experimental feature require extending enumerations of the core specification. 
-If this is necessary should set the `extend` field to true and list the required
-enumerations to support the experimental feature. These additional enumerations 
-will updated the specification with the appropriate values.
+If this is necessary, create a new enum with the ``extend`` field set to true 
+and list the required enumerations to support the experimental feature. These 
+additional enumerations will updated the specification with the appropriate 
+values.
 
 
 Naming Convention

scripts/parse_specs.py

@@ -813,7 +813,13 @@ def refresh_enum_meta(obj, header, meta):
 
 
 def handle_enum_extensions(d, specs, header, meta):
-    matching_enum = [obj for s in specs for obj in s['objects'] if obj['type'] == 'enum' and d['name'] == obj['name']][0]
+    matching_enums = [obj for s in specs for obj in s['objects'] if obj['type'] == 'enum' and d['name'] == obj['name'] and not obj.get('extend')]
+    if len(matching_enums) != 1:
+        raise Exception(f"""
+Could not find matching enum to extend {d['name']}. 
+You should check the ordinal value of the headers to ensure that the enum
+is defined before you extend it.""")
+    matching_enum = matching_enums[0]
     matching_enum['etors'].extend(d['etors'])
 
     ## Refresh metadata