References
  
  [Bar] Barakat, M., Spectral Filtrations via Generalized Morphisms, , , (), ,
  ((arxiv.org/abs/0904.0240)).
  
  [Bar10]   Barakat,   M.,   The   homalg   package   --   GAP4   (2007-2010),
  ((http://homalg.math.rwth-aachen.de/index.php/core-packages/homalg-package)).
  
  [BR06] Barakat, M. and Robertz, D., mathtthomalg: First steps to an abstract
  package  for  homological  algebra,  in  Proceedings  of  the  X  meeting on
  computational  algebra  and  its  applications  (EACA 2006), Sevilla (Spain)
  (2006),                                                              29--32,
  ((http://homalg.math.rwth-aachen.de/maple/homalg_eaca06.pdf)).