For the record, I've briefly looked into this. The initially reported build error can be fixed by applying https://gitlab.math.univ-paris-diderot.fr/cduce/cduce/commit/020feca404095bcc08ebfa6deebaecbbc7399852. There are, however, some other build errors due to the reference to OCaml source files in Makefile. The upstream cduce-next branch can be built with Debian's OCaml 4.05. I've asked upstream (Giuseppe Castagna) to make a new release and I was told that they will do it soon.
Best, Andy
