code: def common_substring(s1, s2):
      
      chunks = []
      path1 = splitall(s1)
      path2 = splitall(s2)
      for (dir1, dir2) in zip(path1, path2):
          if dir1 != dir2:
              break
          chunks.append(dir1)
      return os.path.join(*chunks)
docs: Returns the longest common substring to the two strings, starting from the
      left.