program gray_code;
    loop for n in [0..31] do
        print(
            lpad(str n, 2) + ": " +
            lpad(bits n, 5) + " => " +
            lpad(bits(g := gray_encode(n)), 5) + " => " +
            lpad(bits(d := gray_decode(g)), 5) + ": " +
            lpad(str d, 2)
        );
    end loop;

    proc gray_encode(n);
        return n bit_xor (n div 2);
    end proc;

    proc gray_decode(n);
        loop
            init r := 0;
            step n div:= 2;
            until n=0 do r bit_xor:= n;
        end loop;
        return r;
    end proc;

    op bits(n);
        b := '' +/ [[str (n mod 2), n div:=2](1) : until n=0];
        return reverse(b);
    end op;
end program;