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We assume the election URL is $PREFIX/elections/$UUID/election.json,
and we use URLs relative to $PREFIX elsewhere. For compatibility with
reverse-proxies.

The CAS protocol requires passing around absolute URLs, which cannot
be computed correctly by Eliom if it is behind a reverse proxy. This
option allows one to specify how to post-process URLs computed by
Eliom to match reverse-proxy configuration.

The credential is taken from the file given to --privkey (if any).

 * more comprehensive list of system prerequisites
 * update opam-bootstrap.sh documentation

Because OPAM deletes OPAMROOT in some circumstances...