@Override
    public void visitMembers(ObjectVisitor visitor) {
        super.visitSuperMembers(visitor);
        visitor.visit("field", getFieldName());
    }
Constructs an instance of this class with given field name
and lookup argument.

@param field The field to retrieve.
@param arg Expression evaluating to the lookup argument.
searchlib/src/main/java/com/yahoo/searchlib/expression/InterpolatedDocumentFieldLookupNode.java