Shorter proof for ∃, also add proof for ∀.

2 jobs for !319 with ralf/list_in_dec in 5 minutes and 9 seconds (queued for 6 seconds)
latest detached
Status Name Job ID Coverage
  Build
passed build-coq.8.10.2 #142520
fp

00:05:09

passed build-coq.8.13.2-mr #142519
fp

00:04:52