During a [discussion at the n-category theory cafe][1] Stephen Harris sent me this [excellent expository article by Galuberman][2] which goes into a bit more depth than wikipedia.


  [1]: http://golem.ph.utexas.edu/category/2009/02/last_person_standing.html#more
  [2]: http://www.mat.unb.br/~matcont/16_5.ps