@book {BrTop, 
    author = "Brown, Ronald",
     title = "Topology: a geometric account of general topology,
              homotopy types, and the fundamental groupoid",
 publisher = "Ellis Horwood, Chichester", 
      year = "1988",
}

@book {BrTopGpd, 
    author = "Brown, Ronald",
     title = "Topology and groupoids",
 publisher = "www.groupoids.org, Deganwy", 
      year = "2006",
}

@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-extensions of groups",
   journal = "Georgian Math. J.", 
    volume = "9",
      year = "2002", 
     pages = "623-644",
}

@book {HiGpd,
    author = "Higgins, Philip",
 booktitle = "Categories and Groupoids",
publisher  = "Van Nostrand Reinhold, London",
year       = "1971",
}

@article {HiJLMS,
    author = "Higgins, Philip", 
     title = "The fundamental groupoid of a graph of groups",
   journal = "J. London Math. Soc.", 
    volume = "13",
      year = "1976", 
     pages = "145-149",
}

@phdthesis{emma-thesis,
author     = "Moore, Emma",
title      = "Graphs of groups: 
              word computations and free crossed resolutions",
type       = "Ph.{D}.~thesis",
year       = "2001",
school     = "University of Wales, Bangor",
}

@book {Serre,
    author = "Serre, J.",
     title = "Trees",
publisher  = "Springer-Verlag, Berlin",
year       = "1980",
}