program longest_common_substring;
    print(lcs("thisisatest", "testing123testing"));

    proc lcs(s1, s2);
        if #s1 < #s2 then [s1, s2] := [s2, s1]; end if;

        loop for l in [#s2, #s2-1..1] do
            loop for s in [1..#s2-l+1] do
                if (substr := s2(s..s+l)) in s1 then
                    return substr;
                end if;
            end loop;
        end loop;

        return "";
    end proc;
end program;