input: Write a python function to replace each character in a string with the next character in the alphabet, wrapping 'z' to 'a'.
  assert replace_char('abc') == 'bcd'
  assert replace_char('xyz') == 'yza'
  assert replace_char('hello') == 'ifmmp'
output: def replace_char(s):
      return ''.join(chr((ord(c) - 97 + 1) % 26 + 97) if 'a' <= c <= 'z' else c for c in s)