new_contents: def checkio(numbers):
      paths = [[numbers[0]]]
      while True:
          p = paths.pop(0)
          for c in set(numbers)-set(p):
              if sum(x!=y for x, y in zip(str(p[-1]), str(c))) == 1:
                  if c == numbers[-1]:
                      return p + [c]
                  paths.append(p + [c])
  
subject: Create digits-doublets.py