sub EXISTS {
    my( $pre, $key, $post ) = unpack $mask{ $_[SELF] }, $_[KEY];
    die "Prefix mismatch $pre ne $pre{ $_[SELF] }"
        unless $pre eq $pre{ $_[SELF] };

    return
        defined $_[SELF]{ $key }
            and $_[SELF]{ $key }[ 0 ] =~ m[(...\c@)\Q$post\E\xff]s;
}