proposition: Lemma joinA f1 f2 f3 : f1 \+ (f2 \+ f3) = (f1 \+ f2) \+ f3.
proof: Proof. by rewrite (joinC f2) joinCA joinC. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)