X Y Z : X + (Y + Z) = X + Y + Z. Proof. symmetry. apply path_universe_uncurried. apply equiv_sum_assoc. Qed.
From HoTT Require Import TruncType ExcludedMiddle abstract_algebra. From HoTT Require Import Universes.Smallness. From HoTT Require Import Spaces.Nat.Core Spaces.Card. From HoTT Require Import Equiv.BiInv. From HoTT Require Import HIT.unique_choice. From HoTT.Sets Require Import Ordinals Hartogs Powers GCH AC.