∀ {α : Type u} {β : α → Type v} {x : BEq α} {x_1 : Hashable α} {m : Std.DHashMap α β} [inst : LawfulBEq α] {k : α}
  {f : Option (β k) → Option (β k)}, k ∈ m → (f (m.get? k)).isSome = true → (m.alter k f).size = m.size