∀ {α : Type u} {β : Type v} {x : BEq α} {x_1 : Hashable α} {m : Std.ExtHashMap α β} [inst : EquivBEq α]
  [inst_1 : LawfulHashable α] [inst_2 : Inhabited α] {k k' : α}, (k == k') = true → m.getKey! k = m.getKey! k'