切片 (slice)
  s = 'ABCDE'
  a = s[1:4]  # a -> 'BCD'
  a = s[1:]   # a -> 'BCDE'
  a = s[:2]   # a -> 'AB'
  a = s[:]    # a -> 'ABCDE'
  a = s[4:2]  # a -> ''
  a = s[2:10000] # a -> 'CDE' 開始索引/結束索引可以越界
  a = s[::2]  # a -> 'ACE'
  a = s[1::2] # a -> 'BD'
  a = s[::-1] # a -> 'EDCBA'
  a = s[::-2] # a -> 'ECA'
  a = s[4:0:-2] # a ->'EC'
