code: def flatten_list(l):
      
      if (l == []) or (l is None):
          return l
      if isinstance(l[0], list):
          return flatten_list(l[0]) + flatten_list(l[1:])
      return l[:1] + flatten_list(l[1:])
docs: Unpacks lists in a list:
  
          [1, 2, [3, 4], [5, [6, 7]]]
  
      becomes
  
          [1, 2, 3, 4, 5, 6, 7]
  
      http://stackoverflow.com/a/12472564/3381305