I have an extremely simple goal to prove:
HEq (fun px rd => match px, rd with | Sum.inr _ppos, dir => dir) fun x => idThe reason the match happens is because the type of px is (PEmpty ⊕ t) and dir is a variable whose type depends on the value of px. So this is a heterogeneous equality between a dependent function that matches on its first argument but doesn't use it - it does so only to teach the compiler, and a non-dependent function (basically const).
I'm in tactic mode but cannot use funext here, because it seems to only apply to nondependent functions. I basically want to do a substitution, but don't know how.
