<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def energy_upperbound(self, spins):
 """A formula for an upper bound on the energy of Theta with spins fixed. Args: spins (dict):
 Spin values for a subset of the variables in Theta. Returns: Formula that upper bounds the energy with spins fixed. """

        subtheta = self.theta.copy()
        subtheta.fix_variables(spins)

        # ok, let's start eliminating variables
        trees = self._trees

        if not trees:
            # if there are no variables to eliminate, then the offset of
            # subtheta is the exact value and we can just return it
            assert not subtheta.linear and not subtheta.quadratic
            return subtheta.offset

        energy = Plus(self.message_upperbound(trees, {}, subtheta), subtheta.offset)

        return energy