text: a = [[1,2],[3,4,5],[6],[7,8,9,10]]
  
  r=[[]]
  for x in a:
      t = []
          for y in x:
              for i in r:
                  t.append(i+[y])
      r = t
  
  print r
  
  -------------------------------
  [[1, 3, 6, 7], [2, 3, 6, 7], [1, 4, 6, 7], [2, 4, 6, 7], [1, 5, 6, 7], [2, 5, 6, 7], [1, 3
  , 6, 8], [2, 3, 6, 8], [1, 4, 6, 8], [2, 4, 6, 8], [1, 5, 6, 8], [2, 5, 6, 8], [1, 3, 6, 9
  ], [2, 3, 6, 9], [1, 4, 6, 9], [2, 4, 6, 9], [1, 5, 6, 9], [2, 5, 6, 9], [1, 3, 6, 10], [2
  , 3, 6, 10], [1, 4, 6, 10], [2, 4, 6, 10], [1, 5, 6, 10], [2, 5, 6, 10]]
  