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