It stops here. Minion B dissapears and everything else still worked because it was the result of the rush minion that the game/code doesn’t care anymore about. It’s only the “Summon the 3/3 rush” that is affected.
Or this yes. That’s also a good idea. I said nothing because (at least in Cmath) it’s easier to just put “Nothing” and is finished
You do realize that your UNDO BC in this case worthless? Because effect from BC already happened?
If played BC which spawns 1/1 Rush with Divine Shield and that via chain of events wiped your entire board, and all your UNDO does is remove my 1/1 - then your UNDO is trash.
Well yes but I would not use this card in such cases. I would use it when my opponent plays cards like Reno or the new quest warlock. The idea of the card is to be a tech against very strong battlecries not to go back in time.
That seems like a bad play.
I think the best questions here are what is the inverted form of XYZ battlecry.
So ‘gain charge if you opponent has a weapon’ means lose charge (i.e. no effect)
Summon token with id 2048 = remove if exists token with id 2048
Etc
Lets talk about quest Warlock.
Warlock plays Flame Imp and deals 3 dmg to himself (progressing on Quest chain)
Warlock then nuked Imp with Spirit Bomb (again progressing on Quest chain)
Warlock then plays 0 mana Shadow Rez and gets Imp back
Warlock plays Imp again. Spirit Bomb it again.
Warlock plays 2nd Shadow rez.
Warlock plays Imp again
Exactly which of this steps UNDO will stop and or revert?
Inverted form of deal 3 damage to self = heal 3 damage to self
bad play, good play is not the question - the question is there must be code to handle this in either case and what that code must do 
It would cancel the last Imp. It would heal the warlock with 3.
Thats BARELY stops warlock from going forward.
And if Quest chain is already completed at that point, then you just healing warlock for 3.
Also interrupt battlecries being interrupted.
Agreed, and that’s what I mean about inverting battlecry. You add an inverted flag to each battlecry (true/false) and adjust logic depending on that parameter. Most would be very straightforward, then things like Zeph… well the designer just makes a call - I’d lean towards delete the perfect card if opponent didn’t play it.
Like Hurmugar said, it’s more like a revert in this cases. “Cancel” is more proper IMO because there are battlecries like Tamsin’s “Battlecry: For the rest of the game, damage you take on your turn damages your opponent instead” and the proposed card should actually cancel the battlecry.
Well the point is to use the card exactly after the quest reward is played, not to stop other battlecries.
Tour Guide is one card that makes something like this a logistical nightmare.
Warlock plays Tour Guide, then taps triggering the 4 pip of Runed Mithril Rod, the discount lets the Warlock play 4 spells for 0 mana, triggering two parts of their Questline.
What would happen if you negated that Tour Guide?
Yeah that basically equals to Delete all Quests from the game.
That’s not a viable way to create a tech card.
No need to make it complex the inversion of Tamsin is that damage you take on your turn causes you damage.
Your opponent’s next hp costs 2 (or regular amount of not 2)
Again - this approach would require to wrote a 100% more code for inverted version of EACH battlecry and have players to remember that.
But the Warlock already got the benefit, so the card didn’t do what it said it would.
Only way i can imagine this is. Opponent plays battlecry, you would be given space to counter the battlecry during the opponents turn lets say by using mana that u left from previous round. Could be interesting mind games.
Agree lots of work but not 100%, most functions could be solved with a ternary.