code: def Unregister(self, name):
      
      precondition.AssertType(name, Text)
  
      try:
        del self._constructors[name]
      except KeyError:
        raise ValueError("Constructor with name '%s' is not registered" % name)
docs: Unregisters a constructor.
  
      Args:
        name: A name of the constructor to unregister.
  
      Raises:
        ValueError: If constructor with specified name has never been registered.