∀ {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 k' : α} {f : Option β → Option β},
  k' ∈ Std.DHashMap.Const.alter m k f ↔
    if (k == k') = true then (f (Std.DHashMap.Const.get? m k)).isSome = true else k' ∈ m