Coba jelasin Lihat juga dari Kumpulan homotopi dalam bahasa melayu dong

Ronald Brown, `Groupoids and crossed objects in algebraic topology', Homology, Homotopy and Applications, 1 (1999) 1–78.

Ronald Brown, Philip J. Higgins, Rafael Sivera, Nonabelian algebraic topology: filtered spaces, crossed complexes, cubical homotopy groupoids, EMS Tracts in Mathematics Vol. 15, 703 pages, European Math. Society, Zürich, 2011.