“…If M is of (2, 2)-type and contains a submodule of (1, 1)-type, then M is isomorphic to the module M 2 (1, r, η), where r ∈ Z 2 , η = ∞ or η ∈ k. Using the pullback diagrams given in [8, p. 15], one can describe the structures of the modules M 2 (1, r, η), r ∈ Z 2 , η = ∞ or η ∈ k. If M is of (2, 2)-type and does not contain any submodule of (1, 1)-type, then M is isomorphic to a submodule of 2P (1, r) for some r ∈ Z 2 (see [8]). Using the structure of P (1, r), one can prove that M has the structure described in (11) below (see Lemma 2.30).…”