TITLE: Any references for Wall's Lemma of exactness?
QUESTION [5 upvotes]: At title goes, I read the following lemma in Bredon's book (GTM139 Topology and Geometry Page 189). But I find that I have never seen it in any other texts on algebraic topology or homological algebra. So I would like to ask the references for it.

REPLY [5 votes]: See
Wall, C. T. C. "On the exactness of interlocking sequences." Enseignement math 12 (1966): 95-100
https://www.e-periodica.ch/cntmng?pid=ens-001:1966:12::36
Hardie, K. A., and K. H. Kamps. "Exact sequence interlocking and free homotopy theory." Cahiers de topologie et géométrie différentielle catégoriques 26.1 (1985): 3-31
http://www.numdam.org/article/CTGDC_1985__26_1_3_0.pdf