theorem lmemeq1d (_G: wff) (_a1 _a2 l: nat): $ _G -> _a1 = _a2 $ > $ _G -> (_a1 IN l <-> _a2 IN l) $;
_G -> _a1 = _a2
_G -> l = l
_G -> (_a1 IN l <-> _a2 IN l)