code: def flat(l):
      
      newl = []
      for i in range(len(l)):
          for j in range(len(l[i])):
              newl.append(l[i][j])
      return newl
docs: Returns the flattened version of a '2D' list.  List-correlate to the a.flat()
  method of NumPy arrays.
  
  Usage:    flat(l)