op .longest(a, b);
      return if #a > #b then a else b end;
    end .longest;

    procedure lcs(a, b);
      if exists empty in {a, b} | #empty = 0 then
        return empty;
      elseif a(1) = b(1) then
        return a(1) + lcs(a(2..), b(2..));
      else
        return lcs(a(2..), b) .longest lcs(a, b(2..));
      end;
    end lcs;