1. Alp, M.: GAP, crossed modules, cat1-groups: applications of computational group theory. PhD thesis, University of Wales, Bangor (1997)
2. Alp, M., Wensley, C.D.: Enumeration of cat1-groups of low order. Internat. J. Algebra Comput. 10, 407–424 (2000)
3. Alp, M., Wensley, C.D.: XMod: crossed modules and cat1-groups in GAP, version 2.12. http://www.maths.bangor.ac.uk/chda/xmod/ (2008)
4. Baez, J.C., Lauda, A.D.: Higher-dimensional algebra V: 2-groups. Theory Appl. Categ. 12, 423–491 (2004)
5. Brown, R.: Topology and Groupoids. Booksurge PLC, North Charleston (2006)