def empty_is_empty(asker): return asker.reply(answer=T.yes())
def return_yes(asker): return asker.reply(answer=T.yes())
def no_to_yes(asker): return asker.reply(answer=T.yes())
def opposite(a): if a.head == T.yes.head: return T.no() elif a.head == T.no.head: return T.yes() return T(opposite.head, a=a)