'Prune unsat paths.'

def prune(self):

    for state in list(self.active):
        if (not state.satisfiable()):
            self._hierarchy.unreachable_state(state)
            self._hierarchy.simplify()
            self.active.remove(state)
            self.pruned.append(state)
    for state in list(self.spilled):
        if (not state.satisfiable()):
            self._hierarchy.unreachable_state(state)
            self._hierarchy.simplify()
            self.spilled.remove(state)
            self.pruned.append(state)
