d its current value as primed [17]. 4 Bi-Abduction In this section we introduce our bi-abduction algorithm for discovering missing information in precondition. The bi-abduction extends previous works =-=[2, 7, 23]-=- with more power to work over our combined domain. Given ? and ?1, bi-abduction aims to find the anti-frame ? ? and frame part ?2 such that ? ? [? ? ] ? ?1 ? ?2 where ? and ?1 can be considered as the