Merge pull request #774 from yallop/plus-callback-fix #232

Job	Run time
Tests (ubuntu-latest, 4.03.0)	5m 52s
Tests (ubuntu-latest, 4.04.0)	5m 49s
Tests (ubuntu-latest, 4.05.0)	5m 58s
Tests (ubuntu-latest, 4.06.0)	6m 4s
Tests (ubuntu-latest, 4.07.0)	6m 9s
Tests (ubuntu-latest, 4.08.0)	3m 28s
Tests (ubuntu-latest, 4.09.0)	4m 3s
Tests (ubuntu-latest, ocaml-variants.4.11.2+fp+flambda)	4m 3s
Tests (ubuntu-latest, 4.11.1)	3m 45s
Tests (ubuntu-latest, 4.12.0)	3m 45s
Tests (ubuntu-latest, 4.13.1)	3m 46s
Tests (ubuntu-latest, 4.14.0)	4m 6s
Tests (ubuntu-latest, 5.0.0)	3m 54s
Tests (ubuntu-latest, 5.1.0)	3m 51s
Tests (ubuntu-latest, 5.2.0)	4m 3s
Tests (windows-latest, 4.13.1)	6m 22s
Tests (macos-latest, 4.13.1)	3m 51s
	1h 18m 49s

Provide feedback