my $user_pfx = $self->{-user_pfx} = [];
for (@$altid) {
# $_ = 'serial:gmane:/path/to/gmane.msgmap.sqlite3'
+ # note: Xapian supports multibyte UTF-8, /^[0-9]+$/,
+ # and '_' with prefixes matching \w+
/\Aserial:(\w+):/ or next;
my $pfx = $1;
push @$user_pfx, "$pfx:", <<EOF;