“…Example 2.3. Further non-trivial examples of elements in S(D 2 ; 2, 1) related to the theory of Hilbert modules in H 2 can be obtained from Theorem 3.3 in Wu-S-Yang [15]. P(D 2 ; 2, 1) and Q(D 2 ; 2, 1) are closed under composition of elements in Q(D 2 ; 2, 1) in the following sense (cf.…”