In the context of 706.7a, how do you determine what “that part of the ability” is? The current example going around is a Thespian's stage that copies a Cavern of Souls. I've seen it said that this results in a Cavern that can be tapped for unusable colored mana, but I'm not exactly sure how we get to that point. I'm aware of four other possible interpretations of what happens in the specific case, but I'm much more interested in the process than the example, so I won't list them here.

From the Gatecrash update bulletin:

706.7a

Say you have a pair of linked abilities, and one of them asks you to choose a value or name a card, and the second one refers to that choice. This rule explains that if the first ability is copied, the choice isn't remembered and can't be used for other abilities that may copy it later. For example, if a creature becomes a copy of Voice of All (as opposed to entering the battlefield as a copy of it, in which case you can choose a color), there's no choice of color for the copy, so its protection ability doesn't do anything. Specifically, the rule said “If an ability refers to an undefined choice, that part of the ability won't do anything.” But “won't do anything” was pretty vague. For example, let's say you have a Thespian's Stage that's a copy of Cavern of Souls. No choice of creature type was made for Thespian's Stage, so that part of the ability won't do anything. I imagine four reasonable interpretations of this:

The restriction of having to spend that mana on a spell of a particular creature type doesn't apply. You can spend this mana on any spell, and it can't be countered.
The whole last sentence doesn't apply. You can spend the mana on any spell. It can be countered.
The whole ability doesn't apply. You can't activate it.
The effect doesn't apply. You can activate it, but it won't produce mana.

So, we're cleaning up the rule a bit to simply say the choice of creature type for the Thespian's Stage is undefined. Also, rule 106.6 clarifies what can be done with mana that can be spent only on a restriction based on an undefined choice. (It can't be spent.)

However, doing a comparison of the October 2012 CR to the February 2013 CR, there were no changes to 706.7a at all. I assume this was an oversight, and the “cleaned up” version of the rule will hopefully make it into the next CR update to make this more clear.

Ah. Silly me looking at the February CR alone to find the answer. As a matter of course, are these kinds of omissions common enough that I should make a practice of consulting the most recent update bulletin as well as the most recent CR, or is this an edge case?

It's happened one other time that I know of, so it's not normally a problem. Never hurts to do some quick research, though. :)

I am almost sure that the answer is no. But, can a Thespian's Stage that copied a Cavern of Souls be used to cast an uncounterable Nameless Race?

Nope. You can tap it for colorless (Cavern's 1st ability), and spend that mana - but the uncounterable aspect only comes from the second ability.

And, as we've learned - that mana can't be spent. You can activate the 2nd ability, and add any color of mana to your pool - but you can't spend it. “that spell can't be countered” is only applicable when you spend that mana … Quite the conundrum, eh?

Here's one “combo” that does work … for what it's worth. With Omnath, Locus of Mana on the battlefield, you can keep adding G to your mana pool, making Omnath ever bigger. And, since you can't spend it, you won't be tempted to shrink your Omnath! (Disclaimer: there MIGHT be better combos to explore…)

Edited Scott Marshall (Feb. 22, 2013 05:48:43 PM)

Me final question: I looked today and I don't see a new CR since Feb. 1, so what is the new wording of 706.7a?

Rules docs are updated with each set, not with each undiscovered typo. Wait for Dragon's Maze…