∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} {β : α → Type v} {m : Std.ExtDHashMap α β} [inst : LawfulBEq α] {k k' : α}
  {f : Option (β k) → Option (β k)},
  (m.alter k f).contains k' = if (k == k') = true then (f (m.get? k)).isSome else m.contains k'