* {{cite book |last=May |first=J. Peter |author-link=J. Peter May|title=A Concise Course in Algebraic Topology |year=1999 |publisher=[[University of Chicago Press]] |url=http://www.math.uchicago.edu/~may/CONCISE/ConciseRevised.pdf |access-date=2008-09-27 |name-list-style=vanc }} Section 2.7 provides a category-theoretic presentation of the theorem as a colimit in the category of groupoids.