input: Write a function to convert all characters of a given string to lowercase. You are given a string and you need to return a new string where all the characters are in lowercase. If the input string is empty, return an empty string.
  assert to_lower("HELLO") == "hello"
  assert to_lower("Hello, World!") == "hello, world!"
  assert to_lower("PYTHON") == "python"
  assert to_lower("") == ""
  assert to_lower("123ABC") == "123abc"
output: def to_lower(s):
      return s.lower()