def to_upper_case(string)
      assert_type string, :String, :string
      Sass::Script::Value::String.new(Sass::Util.upcase(string.value), string.type)
    end
def to_upper_case(string)
      assert_type string, :String, :string
      Sass::Script::Value::String.new(Sass::Util.upcase(string.value), string.type)
    end
Converts a string to upper case.

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

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