sub merge {
    my ($s1, $s2) = @_;

    my $pos = length($s1) <= length($s2) ? 0 : length($s1) - length($s2);

    $pos++ while substr($s1, $pos) ne substr($s2, 0, length($s1) - $pos);

    substr($s1,$pos) = $s2;
    return $s1;
}