old_contents: """
  Derive a list of impossible differentials.
  """
  
  import ast
  import sys
  
  def parse(line):
      return ast.literal_eval(line)
  
  def main():
      if len(sys.argv) != 3:
          print("usage: ./find_ids.py [forward differentials file] [backward differentials file]", file=sys.stderr)
          sys.exit(1)
  
      forward_diffs = []
      with open(sys.argv[1]) as f:
          for i, forward_rounds, xss in map(parse, f):
              forward_diffs.append((i, forward_rounds, [set(xs) for xs in xss]))
  
      backward_diffs = []
      with open(sys.argv[2]) as g:
          for i, backward_rounds, yss in map(parse, g):
              backward_diffs.append((i, backward_rounds, [set(ys) for ys in yss]))
  
      # truncate first round of backward differential
      # by comparing last round of forward differential and second last
      # round of backward differential
      ids = []
      for i, forward_rounds, xss in forward_diffs:
          for j, backward_rounds, yss in backward_diffs:
              if xss[-1].isdisjoint(yss[-2]):
                  backward_rounds -= 1
                  print((i, forward_rounds, backward_rounds, j))
  
  if __name__ == "__main__":
      main()
  
new_contents: """
  Derive a list of impossible differentials.
  """
  
  import ast
  import sys
  
  def parse(line):
      return ast.literal_eval(line)
  
  def main():
      if len(sys.argv) != 3:
          print("usage: ./find_ids.py [forward differentials file] [backward differentials file]", file=sys.stderr)
          sys.exit(1)
  
      forward_diffs = []
      with open(sys.argv[1]) as f:
          for i, forward_rounds, xss in map(parse, f):
              forward_diffs.append((i, forward_rounds, [set(xs) for xs in xss])
  
      backward_diffs = []
      with open(sys.argv[2]) as g:
          for i, backward_rounds, yss in map(parse, g):
              backward_diffs.append((i, backward_rounds, [set(ys) for ys in yss])
  
      # truncate first round of backward differential
      # by comparing last round of forward differential and second last
      # round of backward differential
      ids = []
      for i, forward_rounds, xss in forward_diffs:
          for j, backward_rounds, yss in backward_diffs:
              if xss[-1].isdisjoint(yss[-2]):
                  backward_rounds -= 1
                  print((i, forward_rounds, backward_rounds, j))
  
  if __name__ == "__main__":
      main()
  
subject: Revert "Correct typo."