private Object evaluate(DocumentOperation op) {
        if (hasData(op))
            return evaluateForDataLookup(op);
        else // Simplify to just false since we can't progress here?
            return op.getId().getDocType().equals(type) ? op : false;
    }
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