The 3-by-3 lemma for regular Goursat categories

Stephen Lack
2004 Homology, Homotopy and Applications  
We prove a 3 × 3 lemma for regular Goursat categories. The classical "3 × 3 lemma" in abelian category theory involves short exact sequences. In more general contexts -in particular in a category which is not pointed -it makes sense to replace short exact sequences by "exact forks": these are diagrams
doi:10.4310/hha.2004.v6.n1.a1 fatcat:dz6if7zwbzgrtl4xifzqsx3w6q