code: def insert_meta_fields_into_existing_schema(graphql_schema):
      
      root_type_name = graphql_schema.get_query_type().name
  
      for type_name, type_obj in six.iteritems(graphql_schema.get_type_map()):
          if type_name.startswith('__') or type_name == root_type_name:
              # Ignore the types that are built into GraphQL itself, as well as the root query type.
              continue
  
          if not isinstance(type_obj, (GraphQLObjectType, GraphQLInterfaceType)):
              # Ignore definitions that are not interfaces or types.
              continue
  
          for meta_field_name, meta_field in six.iteritems(EXTENDED_META_FIELD_DEFINITIONS):
              if meta_field_name in type_obj.fields:
                  raise AssertionError(u'Unexpectedly encountered an existing field named {} while '
                                       u'attempting to add a meta-field of the same name. Make sure '
                                       u'you are not attempting to add meta-fields twice.'
                                       .format(meta_field_name))
  
              type_obj.fields[meta_field_name] = meta_field
docs: Add compiler-specific meta-fields into all interfaces and types of the specified schema.
  
      It is preferable to use the EXTENDED_META_FIELD_DEFINITIONS constant above to directly inject
      the meta-fields during the initial process of building the schema, as that approach
      is more robust. This function does its best to not mutate unexpected definitions, but
      may break unexpectedly as the GraphQL standard is extended and the underlying
      GraphQL library is updated.
  
      Use this function at your own risk. Don't say you haven't been warned.
  
      Properties added include:
          - "_x_count", which allows filtering folds based on the number of elements they capture.
  
      Args:
          graphql_schema: GraphQLSchema object describing the schema that is going to be used with
                          the compiler. N.B.: MUTATED IN-PLACE in this method.