theorem lmemeq2d (_G: wff) (a _l1 _l2: nat): $ _G -> _l1 = _l2 $ > $ _G -> (a IN _l1 <-> a IN _l2) $;
_G -> a = a
_G -> _l1 = _l2
_G -> (a IN _l1 <-> a IN _l2)