input: Write a function to convert all characters of a given string to lowercase. The function should take a string as input and return a new string with all characters in lowercase.
  
  def to_lower(string):
      return string.lower()
  
  assert to_lower("HELLO")=="hello"
  assert to_lower("Hello World")=="hello world"
  assert to_lower("PYTHON")=="python"
  assert to_lower("123ABC")=="123abc"
  assert to_lower("This Is A Test")=="this is a test"
output: def to_lower(string):
      return string.lower()