- allow hierarchical organization of stages (serially for now)
- validate correctness of tree (at composition time, i.e. runtime)
- derive Task from SerialContainer
- fix pimpl_func(), PRIVATE_CLASS declaration in "public" section to allow access in tests
SubTask cannot be instantiated directly, but only its derived classes
that specify the type of operation:
PropagatingAnyWay
PropagatingForward
PropagatingBackward
Connecting
Generator