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("HELlo")=="hello"
  assert to_lower("helLO")=="hello"
  assert to_lower("HELLO WORLD")=="hello world"
  assert to_lower("")==""
output: def to_lower(string):
      return string.lower()