input: Write a function to convert all characters of a given string to lowercase. Provide a demonstration with specific test cases.
  
  def to_lower(string):
      return string.lower()
  
  assert to_lower("HELLO") == "hello"
  assert to_lower("WORLD") == "world"
  assert to_lower("Python") == "python"
  assert to_lower("123ABC") == "123abc"
  assert to_lower("") == ""
output: def to_lower(string):
      return string.lower()