...continue to proceed test-driven
...scheduler internals turn out to be intricate and cohesive,
and thus the only hope is to adhere to strict testing discipline
- decision to favour small memory footprint
- rather use several Activity records to express invocation
- design Activity record as »POD with constructor«
- conceptually, Activity is polymorphic, but on implementation
level, this is "folded down" into union-based data storage,
layering accessor functions on top
