input: Write a function to convert all characters of a given string to lowercase. If the input string is empty, return the string as is.
  assert to_lower("HELLO")=="hello"
  assert to_lower("WORLD")=="world"
  assert to_lower("Hello World")=="hello world"
  assert to_lower("")==""
  assert to_lower("123ABC")=="123abc"
output: def to_lower(s):
      return s.lower()