code: def define(self, id, schema):
          
          self.definitions[id] = schema
          self._schema = None
          return self.ref(id)
docs: Add a schema to the list of definition
  
          :param id: id of the schema.
          :param schema: the schema as a dict or a
                         :class:schemabuilder.primitives.Generic
          :return: reference to schema.
          :rtype: :class:`schemabuilder.schema.Ref`