∀ {α : Type u} {β : α → Type v} [inst : BEq α] (a : α) (fallback : (a : α) × β a)
  (x : Std.DHashMap.Internal.AssocList α β),
  Std.DHashMap.Internal.AssocList.getEntryD a fallback x =
    match x with
    | Std.DHashMap.Internal.AssocList.nil => fallback
    | Std.DHashMap.Internal.AssocList.cons k v es =>
      if (k == a) = true then ⟨k, v⟩ else Std.DHashMap.Internal.AssocList.getEntryD a fallback es