sub mrm_4 {
    # from [bart]'s vec()
    my ($s1, $s2) = @_;
    use bytes;

    my $pos = 0;
    vec( $$s1, $pos, 8 ) ||= vec( $s2, $pos, 8 ) while -1 < ( $pos = index $$s1, '\0', $pos );
}