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