solve_proper: Do not enforce unfolding the head symbol

It is sometimes not desirable to do so.
2 jobs for master in 5 minutes and 19 seconds (queued for 1 minute and 24 seconds)
Status Job ID Name Coverage
  Test
passed #2703
coq
stdpp-coq8.5

00:02:55

passed #2704
coq
stdpp-coq8.6

00:02:24