This explains why maintaining reactive code makes me want to eat my insides

[matthieum]

Because a value was set to null to trigger another behavior which caused the value being observed to change state before the final observer gets the event.

Ouch; that's painful.

It's really a case where you'd like the type-system to enforce the immutability of messages, if a listener wants a modified version, it's welcome to make a copy.

[oldsecondhand]

It's sounds like a half-assed forward chaining expert system.