public function findShell($shell)
    {
        $className = $this->_shellExists($shell);
        if (!$className) {
            $shell = $this->_handleAlias($shell);
            $className = $this->_shellExists($shell);
        }

        if (!$className) {
            throw new MissingShellException([
                'class' => $shell,
            ]);
        }

        return $this->_createShell($className, $shell);
    }