∀ {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 a : α} {fallback v : β},
  Std.DHashMap.Const.getD (m.insert k v) a fallback =
    if (k == a) = true then v else Std.DHashMap.Const.getD m a fallback