Homotopy theory, and change of base for groupoids and multiple groupoids (Q1923797)

In applications of the fundamental groupoid to homotopy theory, it is sometimes useful to consider what happens when one changes the objects. This paper is a survey, without many details, of results obtained by analogous methods in higher-dimensional algebraic systems such as double groupoids and crossed modules. The method analogous to change of objects is called change of base. The applications include various forms of the Van Kampen and Hurewicz theorems.