Merge branch 'robbert/disj_union' into 'master'

Rename multiset "union" into "disjoint union"

Closes #13

See merge request !57
8 jobs for master in 7 minutes and 18 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #24731
fp
build-coq.8.7.0

00:02:41

passed #24730
fp
build-coq.8.7.1

00:02:49

passed #24729
fp
build-coq.8.7.2

00:02:48

passed #24728
fp-timing
build-coq.8.8.0

00:02:46

passed #24727
fp
build-coq.8.8.1

00:02:50

passed #24726
fp
build-coq.8.8.2

00:02:52

passed #24725
fp-timing
build-coq.8.9.0

00:03:13

passed #24724
fp
build-coq.dev

00:04:31