sub id2path {
    my $id = shift;
    # allow ID 0; disallow 12.34
    $id =~ m/^\d{1,9}$/ or return q();
    my $n = length($id) > 6 ? 3 : 2;
    $id = sprintf '%0*s', 3 * $n, $id;
    $_ = "/$_/" for substr($id, $n, $n);
    # or: substr($id, $_, 0, '/') for -2*$n, -$n;
    return $id;
}