proposition: Lemma add_inj a b c : T3nf b -> T3nf c -> a + b = a + c -> b = c.
proof: Proof. move => sb sc h. by rewrite - (sub_pr a sb) - (sub_pr a sc) h. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(fintype bigop)