fact: Definition wb (a : bool) : {b : bool | a = b} := match a as b return a = b -> ({b' : bool | a = b'}) with true => fun (h : a = true) => exist (fun b' => a = b') true h | false => fun (h : a = false) => exist (fun b' => a = b') false h end (refl_equal a).
imports: HB(structures) mathcomp(all_ssreflect all_algebra ring)