input: Write a function to convert all characters of a given string to lowercase. If the string is empty, return the empty string.
  assert to_lower("HELLO")=="hello"
  assert to_lower("Python3.x")=="python3.x"
  assert to_lower("aBcDeF")=="abcdef"
  assert to_lower("")==""
  assert to_lower("123ABC")=="123abc"
output: def to_lower(s):
      return s.lower()