theorem bgcdeq1d (_G: wff) (_a1 _a2 b: nat): $ _G -> _a1 = _a2 $ > $ _G -> bgcd _a1 b = bgcd _a2 b $;
_G -> _a1 = _a2
_G -> b = b
_G -> bgcd _a1 b = bgcd _a2 b