private XelContext newXelContext() {
final Book book = getSheet().getBook();
//VariableResolver
final SSMapResolver resolver = new SSMapResolver(((XelContext)book).getVariableResolver());
final Reference ref = addRef(); //add it back whenever there is new formula
((Ref2d)ref).setUpdatable(true); //and of course a cell is an updatable reference
((Ref2d)ref).setLonglive(true); //cannot be cascade removed (ref to cell)
resolver.set("zkoss.ss.Ref", ref);
return new SimpleXelContext(resolver, ((XelContext)book).getFunctionMapper());
}