proposition: Lemma trivIsetS P Q : P \subset Q -> trivIset Q -> trivIset P.
proof: Proof. by move/subsetP/sub_in2=> sPQ /trivIsetP/sPQ/trivIsetP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)