public function executable($executable, $subpath = null)
    {
        if (!$executable instanceof Closure && !$executable instanceof Executable) {
            throw new App\Exception("Object is not a closure or instance of Coast\App\Executable");
        }
        if (isset($subpath)) {
            $executable = new App\Subpath($executable, $subpath);
        } else if ($executable instanceof App || $executable instanceof Lazy) {
            $executable->isSubapp(true);
        }
        array_push($this->_executables, $executable instanceof Closure
            ? $executable->bindTo($this)
            : $executable);
        return $this;
    }