The 3-by-3 lemma for regular Goursat categories. Lack, Stephen (2004) Homology, Homotopy and Applications
The low-dimensional homology of crossed modules. Gilbert, N.D. (2000) Homology, Homotopy and Applications
Théorie de Schreier supérieure Lawrence Breen (1992) Annales scientifiques de l'École Normale Supérieure
Weak corestriction principle for non-Abelian Galois cohomology. Nguyêñ Quôć Thǎńg (2003) Homology, Homotopy and Applications