Another form of associativity for `opM`.

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

00:02:57

passed #6010
fp-timing
build-coq.8.7.0

00:02:46

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

00:03:03

 
  Deploy
passed #6012
opam

00:00:15

passed #6013
reverse-deps

00:00:14