docs: def extend(self, schema):
          """
          Extend a structured schema by another.
  
          For example extending ``tuple<rstring id, timestamp ts, float64 value>``
          with ``tuple<float32 score>`` results in ``tuple<rstring id, timestamp ts, float64 value, float32 score>``.
  
          Args:
              schema(StreamSchema): Schema to extend this schema by.
  
          Returns:
              StreamSchema: New schema that is an extension of this schema.
          """
          if self._spl_type:
             raise TypeError("Not supported for declared SPL types")
          base = self.schema()
          extends = schema.schema()
          new_schema = base[:-1] + ',' + extends[6:]
          return StreamSchema(new_schema)