Building a Stub-Index on XML Tag

To speed up the LineMarkerProvider on large XML files in my plugin, I would like to implement a stub index on a special XML Tag. I thought using the XmlTagStub and the XmlStubBasedTagElementType is a good idea, but somehow i'm not getting further with it and I would really appreciate some help here (other than the link to the stub-index documentation). Following (in very simple way):

Some Xml structure in foo.xml

<MethodDefinition name="firstMethodName">....</MethodDefinition>

The same xml tag with the same name in boo.xml

<MethodDefinition name="firstMethodName">....</MethodDefinition>

What i'd need is a stub-index with the key firstMethodName and the 2 references to the MethodDefinition XmlTag as values. 

So the first question is, if that is even possible? I could'nt find any example on this. 

So far I've tried to register a stubElementTypeHolder in my plugin.xml

<stubElementTypeHolder class="com.myplugin.stubs.MethodDefinitionTypeHolder"/>

Which looks like 

public interface MethodDefinitionTypeHolder{

    IStubElementType METHOD_DEFINITION = new XmlStubBasedTagElementType("MethodDefinition", MyLanguage.INSTANCE);
}

The stub index in the plugin.xml

<stubIndex implementation="com.myplugin.stubs.MethodDefinitionIndex"/>

...with the code:

public class MethodDefinitionIndex extends StringStubIndexExtension<XmlTag> {
    private static final StubIndexKey<String, XmlTag> KEY = StubIndexKey.createIndexKey("methoddefinition.index");

    @NotNull
    @Override
    public StubIndexKey<String, XmlTag> getKey() {
        return KEY;
    }
}

This is where i did so far and unfortunately I can't get any further. I miss the part, where my XML is parsed, (in the createStub method of XmlStubBasedTagElementType?). Or should I define my own XmlTagStub like MethodDefinitionStub and use that, or just implement my own IStubElementType (maybe as subclass of the XmlStubBasedTagElementType)?

Any hints or examples how to implement somthing like this?

Any help would be appreciated, thanks in advance.

Zsolt.