Add comments to the engine code

[V0ldek]

Is your feature request related to a problem? Please describe.
It's impossible to understand all the checks and edge cases inside the engine code without having someone explain it to you.

Describe the solution you'd like

1. What can be refactored to be more descriptive, should. This is not always possible, especially in recursive due to the control-flow, but some might.
2. Change names of locals to more reflect their domain-specific meaning, and less the implementation details.
3. Add comments where meaning is still not clear.

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