public function attachHelper($helperName, FormHelperInterface $formHelper)
    {
        if ($this->isValidKey($helperName)) {
            if (is_null($formHelperType = $this->resolveHelperType($formHelper))) {
                throw new InvalidArgumentException('Unkown form helper instance given');
            }

            if ($this->hasHelper($formHelperType)) {
                $this->getHelperMap()->get($formHelperType)->set($helperName, $formHelper);
            } else {
                $helperMap = new Map();
                $helperMap->set($helperName, $formHelper);
                $this->getHelperMap()->set($formHelperType, $helperMap);
            }
        } else {
            throw new InvalidArgumentException('Invalid form helper name given');
        }
        return $this;
    }