A user has, in his config:

    ipython: no
    jupyter: /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_devel_vcsn/vcsn/work/vcsn-2.7/build-aux/bin/missing jupyter

This is not helpful in anyway.  In particular, `missing` is unlikely
to still be there when the user fires `vcsn jupyter`.

Leave a big fat nothing instead.