ROIS — Complex definition of "Command Unit List"

Implementing "Command Unit List" is very difficult. It should be simplified

Difficulty of the originally proposed Command Unit List resides in its structure which allows hierarchical parallel command sequences (i.e. parallel command sequences in a parallel command sequence). To simplify the structure, modify it as follows:
1. Rename CommandUnitList to CommandUnitSequence.
2. Make CommandUnit class abstract then derive both CommandMessage and ConcurrentCommands from it.
3. A ConcurrentCommands should hold Branches instead of CommandUnits, then each Branch should hold a sequence of CommandMessages instead of CommandUnit.
By this change, CommandUnitSequence can hold a sequence of CommandUnit and the units are executed sequentially in their order. Some unit in the sequence can be a ConcurrentCommands-class object which holds Branch-class objects and each Branch is executed in parallel. This resolution prohibits a hierarchical parallel command sequence because the Branch-class object cannot hold any ConcurrentCommands objects.