<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def hash_function(self):
 """ Returns the hash function proper. Ensures that `self` is not bound to the returned closure. """

        assert hasattr(self, 'f1') and hasattr(self, 'f2')

        # These are not just convenient aliases for the given
        # attributes; if `self` would creep into the returned closure,
        # that would ensure that a reference to this big, fat object
        # would be kept alive; hence, any hash function would carry
        # around all of the auxiliary state that was created during the
        # generation of the hash parameters.  Omitting `self` ensures
        # this object has a chance to be garbage collected.
        f1, f2, g = self.f1, self.f2, self.g

        def czech_hash(word):
            v1 = f1(word)
            v2 = f2(word)
            return g[v1] + g[v2]

        return czech_hash