input: Write a function to convert all characters of a given string to lowercase. The input is a string, and the output should be the string with all characters in lowercase.
  assert to_lower("HELLO")=="hello"
  assert to_lower("Python")=="python"
  assert to_lower("This is a Test")=="this is a test"
  assert to_lower("")==""
  assert to_lower("12345")=="12345"
output: def to_lower(s):
      return s.lower()