retro() on a `string` creates a range of `dchar`, causing array() pains

[Jakob Ovrum]

Consider this simple function:

        private string findParameterList(string typestr)
        {
                auto strippedHead = typestr.find("(")[1 .. $];
                auto strippedTail = retro(strippedHead).find(")");
                strippedTail.popFront(); // slice off closing parenthesis

                return array(strippedTail);
        }

The type of the return expression is dstring, not string.

What is the most elegant way or correct way to solve this 
friction?

(Note: the function is used in CTFE)