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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 message_upperbound(self, tree, spins, subtheta):
 """Determine an upper bound on the energy of the elimination tree. Args: tree (dict):
 The current elimination tree spins (dict):
 The current fixed spins subtheta (dict):
 Theta with spins fixed. Returns: The formula for the energy of the tree. """

        energy_sources = set()
        for v, subtree in tree.items():

            assert all(u in spins for u in self._ancestors[v])

            # build an iterable over all of the energies contributions
            # that we can exactly determine given v and our known spins
            # in these contributions we assume that v is positive
            def energy_contributions():
                yield subtheta.linear[v]

                for u, bias in subtheta.adj[v].items():
                    if u in spins:
                        yield Times(limitReal(spins[u]), bias)

            energy = Plus(energy_contributions())

            # if there are no more variables in the order, we can stop
            # otherwise we need the next message variable
            if subtree:
                spins[v] = 1.
                plus = self.message_upperbound(subtree, spins, subtheta)
                spins[v] = -1.
                minus = self.message_upperbound(subtree, spins, subtheta)
                del spins[v]
            else:
                plus = minus = limitReal(0.0)

            # we now need a real-valued smt variable to be our message
            m = FreshSymbol(REAL)

            self.assertions.update({LE(m, Plus(energy, plus)),
                                    LE(m, Plus(Times(energy, limitReal(-1.)), minus))})

            energy_sources.add(m)

        return Plus(energy_sources)