performance could be better.

Test_J8_Speed map with 40,000 4-element ArrayLists flattened to one list
with 160,000 elements using:

map.values().stream().flatMap(c ->
c.stream()).collect(Collectors.toList())

       stream      for-loop
Chrome  182 ms      174 ms
FireFox 728 ms      396 ms
Edge    366 ms      186 ms

Java     14 ms       14 ms


So there is no real advantage without parallelism, I think. Still, it is
important to cover this.

- arity worked out
- static vs. nonstatic calling in functional interfaces worked out