public function parseXpath(\DOMXPath $xpath, array $options = array())
 {
     $field_set = Schema\FieldDefinitionSet::create();
     foreach ($xpath->query('./field', $options['context']) as $element) {
         $field_set->add($this->parseField($xpath, $element));
     }
     return $field_set;
 }
 public function __construct()
 {
     $this->fields = FieldDefinitionSet::create();
     $this->options = OptionDefinitionList::create();
 }