input: Write a function to convert all characters of a given string to lowercase.
  
  
  def to_lower(string):
      return string.lower()
  
  assert to_lower("HELLO") == "hello"
  assert to_lower("WORLD") == "world"
  assert to_lower("Python3") == "python3"
  assert to_lower("") == ""
  assert to_lower("123ABC") == "123abc"
  
output: def to_lower(string):
      return string.lower()