sub rel_path($$) {
  my ($target,$start) = @_;
  $start =~ s:/[^/]+:/:;    # remove trailing filename
  my @spath = grep {$_ ne ''} split(/\//,$start); 
  my @tpath = grep {$_ ne ''} split(/\//,$target);

  # strip common start of the path
  while ( @spath && @tpath && $spath[0] eq $tpath[0]) {
    shift @spath;
    shift @tpath;
  }

  return ("../"x(@spath)).join('/', @tpath);
}