Another form of associativity for `opM`.

5 jobs for master in 6 minutes and 4 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed build-coq.8.6.1 #6011
fp-timing

00:02:57

passed build-coq.8.7.0 #6010
fp-timing

00:02:46

passed build-coq.8.7.dev #6009
fp-timing

00:03:03

 
  Deploy
passed opam #6012

00:00:15

passed reverse-deps #6013

00:00:14