input: Write a function to convert all characters of a given string to lowercase. The function should take a string as input and return the converted string.
  assert to_lower("HELLO") == "hello"
  assert to_lower("WORLD") == "world"
  assert to_lower("UPPERCASE") == "uppercase"
  assert to_lower("miXeDcAsE123") == "mixedcase123"
  assert to_lower("") == ""
output: def to_lower(s):
      return s.lower()