I'll respond further shortly.  In the meantime, please notice that
Delegator3.py works the same whether or not your Delegator.__call__()
method is commented out.  That's because you needed to define __call__()
methods in your filters.

We are still suffering from semantic overload.  Let's call the instances
which are chained 'filters' and the Delegator mixin machinery 'nodes' for
the purposes of this discussion (because they act much like the nodes in
a traditional Lisp list).