derived.c: derived.c.pl $(MODULES.pm)
      perl derived.c.pl > $@

##</code><code>##

  tests: source.c.diff

  source.c.diff: source.c.gen source.c
      $(DIFF) $^ > $@ || touch source.c.gen # rerun next time if fail

  source.c.gen: source.c.pl $(MODULES.pm)
      perl source.c.pl > $@

  % make tests
…
  % make tests DIFF=gvimdiff

##</code><code>##

% make tests DIFF=cp