∀ {α : Type u} [inst : BEq α] [inst_1 : Hashable α] {β : Type v} {m : Std.DHashMap.Raw α fun x => β}
  [inst_2 : LawfulBEq α] {k : α} {f : Option β → Option β},
  m.WF →
    k ∈ m → (f (Std.DHashMap.Raw.Const.get? m k)).isNone = true → (Std.DHashMap.Raw.Const.alter m k f).size = m.size - 1