code: def to_dict(self):
          
          schema = super(Schema, self).to_dict()
          schema['$schema'] = "http://json-schema.org/draft-04/schema#"
          if self._id:
              schema['id'] = self._id
          if self._desc:
              schema['description'] = self._desc
          return schema
docs: Return the schema as a dict ready to be serialized.