theorem appendeq1d (_G: wff) (_l11 _l12 l2: nat): $ _G -> _l11 = _l12 $ > $ _G -> _l11 ++ l2 = _l12 ++ l2 $;
_G -> _l11 = _l12
_G -> l2 = l2
_G -> _l11 ++ l2 = _l12 ++ l2