In this paper, we generalize some properties about prefix codes in [1], replacing the condition: the language L is finite with the condition: L satisfies |∪_x∈η(L) x^-1 L| < ∞. Then we give another equivalent characterization on the right dense. At last, we give the element in an ideal K = A ∈ M | AM ∩ P = ∅ of 〈M, ·〉 an equivalent description.