\begin{thebibliography}{BMPW02}

\bibitem[BMPW02]{BrMoPoWe}
R.~Brown, E.J. Moore, T.~Porter, and C.D. Wensley.
\newblock Crossed complexes, and free crossed resolutions for amalgamated sums
  and hnn-extensions of groups.
\newblock {\em Georgian Math. J.}, 9:623--644, 2002.

\bibitem[Bro88]{BrTop}
Ronald Brown.
\newblock {\em Topology: a geometric account of general topology, homotopy
  types, and the fundamental groupoid}.
\newblock Ellis Horwood, Chichester, 1988.

\bibitem[Bro06]{BrTopGpd}
Ronald Brown.
\newblock {\em Topology and groupoids}.
\newblock www.groupoids.org, Deganwy, 2006.

\bibitem[Hig76]{HiJLMS}
Philip Higgins.
\newblock The fundamental groupoid of a graph of groups.
\newblock {\em J. London Math. Soc.}, 13:145--149, 1976.

\bibitem[Moo01]{emma-thesis}
Emma Moore.
\newblock {\em Graphs of groups: word computations and free crossed
  resolutions}.
\newblock Ph.{D}.~thesis, University of Wales, Bangor, 2001.

\bibitem[Ser80]{Serre}
J.~Serre.
\newblock {\em Trees}.
\newblock Springer-Verlag, Berlin, 1980.

\end{thebibliography}