∀ {α : Type u} {β : Type v} {x : BEq α} {x_1 : Hashable α} {m : Std.HashMap α β} [EquivBEq α] [LawfulHashable α] {k : α}
  {f : Option β → Option β}, (m.alter k f).isEmpty = ((m.erase k).isEmpty && (f m[k]?).isNone)