docs: def is_common(schema):
      """
      Is `schema` an common schema.
  
      Args:
          schema: Scheme to test.
  
      Returns:
          bool: ``True`` if schema is a common schema, otherwise ``False``.
  
      """
      if isinstance(schema, StreamSchema):
          return schema.schema() in _SCHEMA_COMMON
      if isinstance(schema, CommonSchema):
          return True
      if isinstance(schema, basestring):
          return is_common(StreamSchema(schema))
      return False