theorem sseq2d (_G: wff) (A _B1 _B2: set): $ _G -> _B1 == _B2 $ > $ _G -> (A C_ _B1 <-> A C_ _B2) $;
_G -> A == A
_G -> _B1 == _B2
_G -> (A C_ _B1 <-> A C_ _B2)