Rafael Ayala Eladio Domínguez Angel R. Francés Antonio Quintero Digital homotopy with obstacles. 2001 46 Electr. Notes Theor. Comput. Sci. http://www.elsevier.com/gej-ng/31/29/23/86/27/show/Products/notes/index.htt#006 db/journals/entcs/entcs46.html#AyalaDFQ01