∀ {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] {k k' fallback : α}, (k == k') = true → m.getKeyD k fallback = m.getKeyD k' fallback