input: Write a function to convert all characters of a given string to lowercase.
  
  assert to_lower("HELLO") == "hello"
  assert to_lower("Python") == "python"
  assert to_lower("OpenAI") == "openai"
  assert to_lower("STRING WITH SPACES") == "string with spaces"
  assert to_lower("") == ""
output: def to_lower(s):
      return s.lower()