References
  
  [hpa10]   homalg  project  authors,  T.,  The  homalg  project  (2003-2010),
  ((http://homalg.math.rwth-aachen.de/)).
  
  [Sch03]  Schenck, H., Computational algebraic geometry, Cambridge University
  Press,  London  Mathematical  Society  Student  Texts, 58, Cambridge (2003),
  xiv+193 pages.