val lcs = fn(s1, s2) {
    var l, r, sublen = 1, 0, 0
    for i of s1 {
        for j in i .. len(s1) {
            if not matching(s2, by=s2s(s1, of=i .. j)): break
            if sublen <= j - i {
                l, r = i, j
                sublen = j - i
            }
        }
    }
    if r == 0: return ""
    s2s s1, of=l .. r
}

writeln lcs("thisisatest", "testing123testing")