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("Python")=="python"
  assert to_lower("123ABC")=="123abc"
  assert to_lower("")==""
output: def to_lower(string):
      return string.lower()