A homotopy double groupoid of a Hausdorff space. Brown, Ronald, Hardie, Keith A., Kamps, Klaus Heiner, Porter, Timothy (2002) Theory and Applications of Categories [electronic only]
A homotopy double groupoid of a Hausdorff space. II: A van Kampen theorem. Brown, Ronald, Kamps, K. H., Porter, Timothy (2005) Theory and Applications of Categories [electronic only]