@manual{ AutoDoc,
  author =           {Gutsche, S. and Horn, M.},
  title =            {{AutoDoc  \texttt{\symbol{45}}  Generate documentation
                      from GAP source code (Version 2017.09.15)}},
  year =             {2017},
  note =             {GAP                   package,                   \href
                      {https://github.com/gap-packages/AutoDoc}
                      {\texttt{https://github.com/}\discretionary
                      {}{}{}\texttt{gap\texttt{\symbol{45}}packages/}\discretionary
                      {}{}{}\texttt{AutoDoc}}},
  printedkey =       {GH17}
}
@manual{ GAPDoc,
  author =           {L{\"u}beck, F. and Neunh{\"o}ffer, M.},
  title =            {{GAPDoc (version 1.6)}},
  organization =     {RWTH Aachen},
  year =             {2017},
  note =             {GAP                   package,                   \href
                      {http://www.math.rwth-aachen.de/~Frank.Luebeck/GAPDoc/index.html}
                      {\texttt{http://www.math.rwth\texttt{\symbol{45}}aachen.de/}\discretionary
                      {}{}{}\texttt{\texttt{\symbol{126}}Frank.Luebeck/}\discretionary
                      {}{}{}\texttt{GAPDoc/}\discretionary
                      {}{}{}\texttt{index.html}}},
  printedkey =       {LN17}
}
@manual{ GitHubPagesForGAP,
  author =           {Horn, M.},
  title =            {{GitHubPagesForGAP  \texttt{\symbol{45}}  Template for
                      easily using GitHub Pages within GAP packages (Version
                      0.2)}},
  year =             {2017},
  note =             {GAP                   package,                   \href
                      {https://gap-system.github.io/GitHubPagesForGAP/}
                      {\texttt{https://gap\texttt{\symbol{45}}system.github.io/}\discretionary
                      {}{}{}\texttt{GitHubPagesForGAP/}}},
  printedkey =       {Hor17}
}
@article{ AlWe,
  author =           {Alp, M. and Wensley, C. D.},
  title =            {Automorphisms  and homotopies of groupoids and crossed
                      modules},
  journal =          {Applied Categorical Structures},
  volume =           {18},
  year =             {2010},
  pages =            {473{\textendash}495},
  printedkey =       {AW10}
}
@book{ BrTop,
  author =           {Brown, R.},
  title =            {Topology:  a  geometric  account  of general topology,
                      homotopy types, and the fundamental groupoid},
  publisher =        {Ellis Horwood, Chichester},
  year =             {1988},
  printedkey =       {Bro88}
}
@book{ BrTopGpd,
  author =           {Brown, R.},
  title =            {Topology and groupoids},
  publisher =        {Booksurge LLC, S.Carolina},
  year =             {2006},
  printedkey =       {Bro06}
}
@article{ BrMoPoWe,
  author =           {Brown, R. and Moore, E. J. and Porter, T. and Wensley,
                      C. D.},
  title =            {Crossed  complexes,  and  free crossed resolutions for
                      amalgamated sums and HNN\texttt{\symbol{45}}extensions
                      of groups},
  journal =          {Georgian Math. J.},
  volume =           {9},
  year =             {2002},
  pages =            {623{\textendash}644},
  printedkey =       {BMPW02}
}
@book{ HiGpd,
  author =           {Higgins, P.},
  title =            {Categories and groupoids},
  publisher =        {Reprints in Theory and Applications of Categories},
  year =             {2005},
  pages =            {1{\textendash}195},
  note =             {\href
                      {http://www.tac.mta.ca/tac/reprints/articles/7/tr7abs.html}
                      {\texttt{http://www.tac.mta.ca/}\discretionary
                      {}{}{}\texttt{tac/}\discretionary
                      {}{}{}\texttt{reprints/}\discretionary
                      {}{}{}\texttt{articles/}\discretionary
                      {}{}{}\texttt{7/}\discretionary
                      {}{}{}\texttt{tr7abs.html}}},
  printedkey =       {Hig05}
}
@article{ HiJLMS,
  author =           {Higgins, P.},
  title =            {The fundamental groupoid of a graph of groups},
  journal =          {J. London Math. Soc.},
  volume =           {13},
  year =             {1976},
  pages =            {145{\textendash}149},
  printedkey =       {Hig76}
}
@phdthesis{ emma-thesis,
  author =           {Moore, E. J.},
  title =            {Graphs  of  Groups: Word Computations and Free Crossed
                      Resolutions},
  school =           {University of Wales, Bangor},
  year =             {2001},
  note =             {\href
                      {http://www.maths.bangor.ac.uk/research/ftp/theses/moore.ps.gz}
                      {\texttt{http://www.maths.bangor.ac.uk/}\discretionary
                      {}{}{}\texttt{research/}\discretionary
                      {}{}{}\texttt{ftp/}\discretionary
                      {}{}{}\texttt{theses/}\discretionary
                      {}{}{}\texttt{moore.ps.gz}}},
  printedkey =       {Moo01}
}
@book{ Serre,
  author =           {Serre, J.},
  title =            {Trees},
  publisher =        {Springer\texttt{\symbol{45}}Verlag, Berlin},
  year =             {1980},
  printedkey =       {Ser80}
}