program balanced_brackets;
    setrandom(0);

    loop for length in [1..10] do
        s := generate length;
        print(balanced s, s);
    end loop;

    op generate(n);
        s := "["*n + "]"*n;
        loop for i in [n*2, n*2-1..2] do
            j := 1 + random(i - 1);
            [s(i), s(j)] := [s(j), s(i)];
        end loop;
        return s;
    end op;

    op balanced(s);
        depth := 0;
        loop for c in s do
            case c of
            ("["):
                depth +:= 1;
            ("]"):
                depth -:= 1;
                if depth<0 then return false; end if;
            end case;
        end loop;
        return depth = 0;
    end op;
end program;