References
  
  [EGN]    Eick,    B.,    G\"ahler,    F.    and   Nickel,   W.,   cryst,
  \URL{http://www.gap-system.org/Packages/cryst.html}.
  
  [Ell] Ellis, G., HAP, \URL{http://hamilton.nuigalway.ie/Hap/www/}.
  
  
  
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