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 lowercase version of the string.
  assert to_lower("HELLO")=="hello"
  assert to_lower("WORLD")=="world"
  assert to_lower("HeLLO_WoRLD")=="hello_world"
  assert to_lower("")==""
  assert to_lower("123")=="123"
output: def to_lower(s):
      return s.lower()