  Abelian track categories can be classified via the third Baues-Wirsching
cohomology of small categories. This approach is used in this paper to compare
and classify different generalisations of the obstruction theory of non-abelian
group extensions, due to Cegarra, Garz\'on and Grandjean, Cegarra, Garz\'on and
Ortega, and Chen, Du and Wang.
