input: Write a function to convert all characters of a given string to lowercase. If the string is empty, return an empty string.
  def to_lower(string):
      return string.lower()
  assert to_lower("HELLO")=="hello"
  assert to_lower("HelloWorld")=="helloworld"
  assert to_lower("123ABC")=="123abc"
  assert to_lower("")==""
  assert to_lower("already lower")=="already lower"
output: def to_lower(string):
      return string.lower()