My thoughts are that Alan stating “Move to combat” and Nathan agreeing meant they have passed priority in the precombat Main Phase and are moving to the Combat Phase. If we're going by what's in the list of official tournament shortcuts, we've only moved to the Beginning of Combat step and not the Declare Attackers step. Alan further indicates this by activating Mutavault before declaring it an attacker, but the problem comes from his immediate declaration of attackers after the activation before Nathan can either respond to the activation or make a possible action during his priority at the end of the Beginning of Combat step. As a result I would give GPE-GRV to Alan, while Nathan receives no penalties. Since Nathan called a judge before too many actions occurred, we can rewind to the point before Alan tapped the Swamp to activate Mutavault. The statement regarding Glimpse the Sun God is mildly suspicious but does not provide enough reason for me to believe he was attempting to cheat by trying to skip past Nathan's priority.

This seems pretty clear to me - in his attempt to be cunning with Glimpse the Sun God, Alan has messed up and moved beyond the point where he can legally animate his Mutavault (it reminds me of the famous “do you have any fast effects?” match).

Looking at it sequentially:
1) Alan says “move to combat”
2) The game advances to Nathan having priority in Beginning of Combat (If Nathan accepts the shortcut)
3) Nathan says “Sure, go ahead”, accepting the shortcut and passing priority.
4) Both players having now passed priority in beginning of Combat, the game advances to Declare Attackers.
5) Alan attempts to animate his Mutavault before taking the action of Declaring Attackers, in Declare Attackers. This is illegal. Nathan calls the judge.

Alan hasn't cheated, but in his attempt at gamesmanship he has messed up and missed his chance at animating the Mutavault.

• GPE-GRV Warning for Alan for his attempted illegal activation.
  
• No penalty for Nathan as he called for a judge immediately.
  
• Rewind the game to the beginning of Declare Attackers, just before the illegtal activiation was attempted. Note that since this is the beginning of Delcare Attackers rather than all the way back to before Alan said “Move to Combat?”, Alan will not be able to activate the Mutavault in time to attack this turn.
  

Edited James Winward-Stuart (Feb. 27, 2014 05:55:20 AM)

Thanks for the discussion this week! A number of you correctly determined that Nathan is correct, Alan had passed the point where he could activate Mutavault before he attacked. Per the tournament shortcuts listed in MTR 4.2 (go read those now if you haven't in the last month!!!), any statements similar to ‘ready for combat’, ‘go to attacks’, etc. are offers to give your opponent priority in the Beginning of Combat step. When Nathan responded with “Sure, go ahead” he passed priority, which moved the game into the Declare Attackers step. At this point Alan tried to activate his mutavault during the Turn-Based Action of declaring attacks.

As a Game Play Error with no more specific category to fall into, this is a GRV. Alan will receive a warning, but Nathan will not receive a warning for Failure to Maintain Game State due to catching the error in a reasonable amount of time. With your head judge's permission the game should be rewound to the point in the game when Alan activated mutavault, and he will continue choosing his attackers from that point.

It is worth noting that asking a few questions in this situation would not be unwarranted. This is likely just a misunderstanding of the way the game progresses by Alan, but if it turns out that he knew about the ‘declare attackers’ tournament shortcut he may be guilty of USC-Cheating.

Make sure to join us tomorrow for our next installment of the Knowledge Pool!