code: def prepend_zeros_to_lists(ls):
      
      longest = max([len(l) for l in ls])
  
      for i in range(len(ls)):
          while len(ls[i]) < longest:
              ls[i].insert(0, "0")
docs: Takes a list of lists and appends 0s to the beggining of each sub_list
      until they are all the same length. Used for sign-extending binary numbers.