docs: def boolean(value):
      """Parse the string ``"true"`` or ``"false"`` as a boolean (case
      insensitive). Also accepts ``"1"`` and ``"0"`` as ``True``/``False``
      (respectively). If the input is from the request JSON body, the type is
      already a native python boolean, and will be passed through without
      further parsing.
      """
      if isinstance(value, bool):
          return value
  
      if not value:
          raise ValueError("boolean type must be non-null")
      value = value.lower()
      if value in ('true', '1',):
          return True
      if value in ('false', '0',):
          return False
      raise ValueError("Invalid literal for boolean(): {0}".format(value))