Demystify `combine` function - Decider

[idugalic]

Explain how to model Commands, Events, and State data types.

• how inheritance/polymorphism (Sealed and Open) can be used to model OR/SUM relationship and why is this important for modeling Commands and Events
• what options do we have to model general AND/PRODUCT relationship, and what kotlin idiomatic options do we have (Pairs)
• explain how combine() function is combining two or more Deciders into one big decider that can just run as a whole. Or you can distribute these deciders via Actors for example and run them in parallel. The decider is a Monoid in respect to the combine() function/operator.
• Mention the principle of duality (boolean algebra) that is present here.
• Why do Command and Event have to be nullable (?) in order to be able to use the combine function in the first place.