Features:
'feature' name = ID
'{'(
    ('action'           '{' action+=Actions (',' action+=Actions)*  '}')? &
    ('dependencies'     '{' dependencies    = Dependencies          '}')? &
    ('children'         '{' children        = Children              '}')?
)'}'

final XtextEditor editor = (XtextEditor)sourcepart;
        final IXtextDocument document = editor.getDocument();
        document.modify(new IUnitOfWork.Void<XtextResource>(){
            public void process (XtextResource resource) throws Exception {
                IParseResult parseResult = resource.getParseResult();
                if(parseResult ==null)
                    return;
            CompositeNode rootNode=(CompositeNode) parseResult.getRootNode();
            LeafNode node = (LeafNode)NodeModelUtils.findLeafNodeAtOffset(rootNode, 0);
            EObject object =NodeModelUtils.findActualSemanticObjectFor(node);

action {
                    act1.set (foo),
                    act2.set (bar),
                    act3.set (baz),
                    act4.set (booze)  //where this is the new action that I add
                }

action {
                            act1.set (foo),
                            act2.set (bar),
                            act3.set (baz)
}

action {
                             act4.set(booze)
       }

Features:
'feature' name = ID
'{'(
    ('action'           '{' actionList=ActionList  '}')? &
    ('dependencies'     '{' dependencies    = Dependencies          '}')? &
    ('children'         '{' children        = Children              '}')?
)'}';

ActionList:
    (action+=Action (',' action+=Action)*)
;