References
  
  [Bro88]  Brown,  R.,  Topology: a geometric account of general topology,
  homotopy  types, and the fundamental groupoid, Ellis Horwood, Chichester
  (1988).
  
  [Bro06]  Brown,  R.,  Topology and groupoids, www.groupoids.org, Deganwy
  (2006).
  
  [BMPW02] Brown, R., Moore, E. J., Porter, T. and Wensley, C. D., Crossed
  complexes,  and  free  crossed  resolutions  for  amalgamated  sums  and
  HNN-extensions of groups, Georgian Math. J., 9 (2002), 623-644.
  
  [Hig76]  Higgins,  P., The fundamental groupoid of a graph of groups, J.
  London Math. Soc., 13 (1976), 145-149.
  
  [Moo01]  Moore, E., Graphs of groups: word computations and free crossed
  resolutions, Ph.{D}.~thesis, University of Wales, Bangor (2001).
  
  [Ser80] Serre, J., Trees, Springer-Verlag, Berlin (1980).
  
  
  
  -------------------------------------------------------