Keywords may not be used as prefixes

Author: gkellogg

lib/context.js

@@ -770,6 +770,12 @@ api.createTermDefinition = ({
         'jsonld.SyntaxError',
         {code: 'invalid term definition', context: localCtx});
     }
+    if(api.isKeyword(mapping['@id'])) {
+      throw new JsonLdError(
+        'Invalid JSON-LD syntax; keywords may not be used as prefixes',
+        'jsonld.SyntaxError',
+        {code: 'invalid term definition', context: localCtx});
+    }
     if(typeof value['@prefix'] === 'boolean') {
       mapping._prefix = value['@prefix'] === true;
     } else {