There is a lacuna in the proof of Lemma 1 of [1]; the projector q is assumed without proof. An alternative, valid proof is as follows.
LEMMA 1. Let S, U0, V0 be real Banach spaces; let A : S → U0 and B : S → V0 be continuous linear maps, whose null spaces are N(A) respectively N(B); let N(A) ⊂ N(B) : let A map S onto U0. Then there exists a continuous linear map C : U0 → V0 such that B = C ° A.