chris@chris-870Z5E-880Z5E-680Z5E:~$ sudo opam update
default    Downloading https://opam.ocaml.org/1.1/urls.txt
Updating ~/.opam/repo/compiler-index ...
Updating ~/.opam/compilers/ ...
Updating ~/.opam/repo/package-index ...
Updating ~/.opam/packages/ ...
The following package has been DELETED:
 - camlp4.4.02.0+2
[WARNING] install and dev-repo are unknown fields in /home/chris/.opam/repo/default/packages/liquidsoap/liquidsoap.1.1.1/opam: is your OPAM up-to-date ?
[WARNING] install is an unknown field in /home/chris/.opam/repo/default/packages/ocurl/ocurl.0.7.2/opam: is your OPAM up-to-date ?
Updating the cache of metadata (~/.opam/state.cache) ...
23 to install | 17 to reinstall | 16 to upgrade | 1 to downgrade | 0 to remove
You can now run 'opam upgrade' to upgrade your system.
chris@chris-870Z5E-880Z5E-680Z5E:~$ sudo opam upgrade
[WARNING] External solver failed with inconsistent return value. Request saved to "/home/chris/.opam/log/solver-error-6572-1.cudf"
Falling back to the internal heuristic.

The dependency ocamlbuild of package variantslib.109.15.03 is not available for your compiler or your OS.
'opam upgrade' failed.

chris@chris-870Z5E-880Z5E-680Z5E:~$ ocaml -version
The OCaml toplevel, version 4.02.0