simplify proof of seqZ_cons

Status Job ID Name Coverage
  Build
canceled #35642
fp
build-coq.8.10.dev

canceled #35648
fp
build-coq.8.7.2

canceled #35647
fp-timing
build-coq.8.8.0

canceled #35646
fp
build-coq.8.8.1

canceled #35645
fp
build-coq.8.8.2

canceled #35644
fp-timing
build-coq.8.9.0

canceled #35643
fp
build-coq.8.9.1

canceled #35641
fp
build-coq.dev