Unwraps the elements of ``value`` using ``SetField.item_type`` and
            returns them in a set

def unwrap(self, value, session=None):
        ''' Unwraps the elements of ``value`` using ``SetField.item_type`` and
            returns them in a set'''
        self.validate_unwrap(value)
        return set([self.item_type.unwrap(v, session=session) for v in value])