@Override
    public Object evaluate(Context context) {
        return evaluate(context.getDocumentOperation());
    }
A document node which returns a document: For accessing document field data in AttributeNode,
where it should be possible to access fields both by the concrete type ("concreteType.fieldName")
and by parent type ("inheritedType.inheritedField").

@author Simon Thoresen Hult
@author bratseth
document/src/main/java/com/yahoo/document/select/rule/DocumentNode.java