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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def Unregister(self, name):
 """Unregisters a constructor. Args: name: A name of the constructor to unregister. Raises: ValueError: If constructor with specified name has never been registered. """

    precondition.AssertType(name, Text)

    try:
      del self._constructors[name]
    except KeyError:
      raise ValueError("Constructor with name '%s' is not registered" % name)