input: Write a function that takes a string as input and returns the lowercase version of the string. Ensure that all characters in the string are converted to their lowercase equivalent.
  
  def str_to_lower(string):
      new_string = ""
  
      for char in string:
          new_string += char.lower()
  
      return new_string
  
  assert str_to_lower("Hello WORLD")=="hello world"
  assert str_to_lower("Python Programming")=="python programming"
  assert str_to_lower("This is a TEST")=="this is a test"
  assert str_to_lower("")==""
  assert str_to_lower("123 ABC")=="123 abc"
output: def str_to_lower(string):
      return string.lower()