'Add a transition to the start of the transition list.
Parameter `transition`: a ready-made transition 3-tuple.
Exception: `DuplicateTransitionError`.'

def add_transition(self, name, transition):

    if self.transitions.has_key(name):
        raise DuplicateTransitionError(name)
    self.transition_order[:0] = [name]
    self.transitions[name] = transition
