References
  
  [Alp97]  Alp,  M.,  GAP,  crossed  modules, cat1-groups: applications of
  computational  group theory, Ph.{D}.~thesis, University of Wales, Bangor
  (1997).
  
  [AW00]  Alp,  M.  and  Wensley, C. D., Enumeration of cat1-groups of low
  order, Int. J. Algebra and Computation, 10 (2000), 407--424.
  
  [Bro82]   Brown,   R.   (Brown,   R.   and   Thickstun,  T.  L.,  Eds.),
  Higher-dimensional  group theory, in Low-dimensional topology, Cambridge
  University  Press,  London  Math.  Soc.  Lecture Note Series, 48 (1982),
  215--238.
  
  [BH78]  Brown,  R.  and  Higgins,  P.  J., On the connection between the
  second  relative  homotopy  group  and some related spaces, Proc. London
  Math. Soc., 36 (1978), 193--212.
  
  [BL87]  Brown, R. and Loday, J. -.L., Van Kampen theorems for diagram of
  spaces, Topology, 26 (1987), 311--335.
  
  [BW95]  Brown, R. and Wensley, C. D., On finite induced crossed modules,
  and  the  homotopy  2-type  of mapping cones, Theory and Applications of
  Categories, 1 (1995), 54--71.
  
  [BW96]  Brown,  R. and Wensley, C. D., Computing crossed modules induced
  by  an  inclusion  of  a  normal subgroup, with applications to homotopy
  2-types, Theory and Applications of Categories, 2 (1996), 3--16.
  
  [BW03]  Brown,  R.  and  Wensley,  C.  D.,  Computation  and homotopical
  applications  of  induced  crossed  modules, J. Symbolic Computation, 35
  (2003), 59--72.
  
  [Ell84]   Ellis,  G.,  Crossed  modules  and  their  higher  dimensional
  analogues, Ph.{D}.~thesis, University of Wales, Bangor (1984).
  
  [ES87] Ellis, G. and Steiner, R., Higher dimensional crossed modules and
  the homotopy groups of (n+1)-ads., J. Pure and Appl. Algebra, 46 (1987),
  117--136.
  
  [Gil90]  Gilbert, N. D., Derivations, automorphisms and crossed modules,
  Comm. in Algebra, 18 (1990), 2703--2734.
  
  [Lod82]  Loday,  J.  L.,  Spaces with finitely many non-trivial homotopy
  groups, J. App. Algebra, 24 (1982), 179--202.
  
  [Moo01]  Moore,  E.  J.,  Graphs  of  Groups: Word Computations and Free
  Crossed Resolutions, Ph.{D}.~thesis, University of Wales, Bangor (2001).
  
  [Nor87]  Norrie, K. J., Crossed modules and analogues of group theorems,
  Ph.{D}.~thesis, King's College, University of London (1987).
  
  [Nor90]  Norrie,  K.  J.,  Actions and automorphisms of crossed modules,
  Bull. Soc. Math. France, 118 (1990), 129--146.
  
  [Whi48]  Whitehead,  J. H. C., On operators in relative homotopy groups,
  Ann. of Math., 49 (1948), 610--640.
  
  [Whi49]  Whitehead,  J.  H.  C.,  Combinatorial homotopy II, Bull. Amer.
  Math. Soc., 55 (1949), 453--496.
  
  
  
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