def to_lower_case(string)
      assert_type string, :String, :string
      Sass::Script::Value::String.new(Sass::Util.downcase(string.value), string.type)
    end
def to_lower_case(string)
      assert_type string, :String, :string
      Sass::Script::Value::String.new(Sass::Util.downcase(string.value), string.type)
    end
Convert a string to lower case,

 @example
   to-lower-case(ABCD) => abcd

 @overload to_lower_case($string)
   @param $string [Sass::Script::Value::String]
 @return [Sass::Script::Value::String]
 @raise [ArgumentError] if `$string` isn't a string