References
  
  [AHU74]  Aho,  A.  V., Hopcroft, J. E. and Ullman, J. D., The Design and
  Analisys of Computer Algorithms, Addison-Wesley, Reading, Mass. (1974).
  
  [DEGKNW02]  Dobkin,  D., Ellson, J., Gansner, E., Koutsofios, E., North,
  S.  and  Woodhull, G., Graphviz - Graph Drawing Programs, AT\&T Research
  and            Lucent            Bell            Labs            (2002),
  (((http://www.research.att.com/sw/tools/graphviz))).
  
  [HU69]  Hopcroft,  J.  E.  and Ullman, J. D., Formal Languages and their
  Relation to Automata, Addison-Wesley, Reading, Mass. (1969).
  
  [KM02]  Kapovich, I. and Myasnikov, A., Stallings foldings and subgroups
  of free groups, J. of Algebra, 248 (2002), 608--668.
  
  [MSW01]  Margolis, S., Sapir, M. and Weil, P., Closed subgroups in pro-V
  topologies  and  the  extension problem for inverse automata, Int. J. of
  Algebra and Computation, 11 (2001), 405--445.
  
  [MMPTV95]  Matz,  O.,  Miller, A., Potthoff, A., Thomas, W. and Valkema,
  E.,  Report  on  the program AMoRe, Christian Albrechts Universit{\"a}t,
  Kiel, 9507 (1995).
  
  
  
  -------------------------------------------------------