如何在Xtend中找出是否使用了Xtext语法中的可选特性？

Model:
    greetings+=Greeting*;

Greeting:
    'Hello' name=ID '!'?;

import org.eclipse.xtext.parser.IParseResult
import org.eclipse.xtext.parser.IParser
import org.eclipse.xtext.resource.XtextResource
import org.eclipse.xtext.util.CancelIndicator

class XtextParser {
    def boolean hasOptionalFeature(String grammarFilePath) {
        val parser = new MyDslStandaloneSetup().createInjectorAndDoEMFRegistration().getInstance(IParser)
        val resource = parser.parse(grammarFilePath)
        val parseResult = resource.contents.head as IParseResult
        val grammar = parseResult.rootASTElement.eResource as XtextResource
        
        for (element : grammar.allContents.toIterable.filter(typeof(AbstractElement))) {
            if (element.optional) {
                return true
            }
        }
        
        return false
    }
}