<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def unregister(self, model):
 """ Unregister a permission handler from the model Parameters model : django model class A django model class Raises ------ KeyError Raise when the model have not registered in registry yet. """

        if model not in self._registry:
            raise KeyError("A permission handler class have not been "
                           "registered for '%s' yet" % model)
        # remove from registry
        del self._registry[model]