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If an ability would produce one or more mana of an undefined type, it produces no mana instead.

Some abilities produce mana based on the type of mana another permanent or permanents “could produce.” The type of mana a permanent could produce at any time includes any type of mana that an ability of that permanent would produce if the ability were to resolve at that time, taking into account any applicable replacement effects in any possible order. Ignore whether any costs of the ability could or could not be paid. If that permanent wouldn’t produce any mana under these conditions, or no type of mana can be defined this way, there’s no type of mana it could produce.

To “tap [a permanent] for mana” is to activate a mana ability of that permanent that includes the {T} symbol in its activation cost. See rule 605, “Mana Abilities.”

An ability that triggers whenever a permanent “is tapped for mana” or is tapped for mana of a specified type triggers whenever such a mana ability resolves and produces mana or the specified type of mana.

A replacement effect that applies if a permanent “is tapped for mana” or tapped for mana of a specific type and/or amount modifies the mana production event while such an ability is resolving and producing mana or the specified type and/or amount of mana.

If an object’s enters-the-battlefield triggered ability or replacement effect refers to X, and the spell that became that object as it resolved had a value of X chosen for any of its costs, the value of X for that ability is the same as the value of X for that spell, although the value of X for that permanent is 0. This is an exception to rule 107.3i.

The tap symbol is {T}. The tap symbol in an activation cost means “Tap this permanent.” A permanent that’s already tapped can’t be tapped again to pay the cost. A creature’s activated ability with the tap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since their most recent turn began. See rule 302.6.

The untap symbol is {Q}. The untap symbol in an activation cost means “Untap this permanent.” A permanent that’s already untapped can’t be untapped again to pay the cost. A creature’s activated ability with the untap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since their most recent turn began. See rule 302.6.

You can change the numeral inside of a mana symbol, but you can’t modify the cost of a spell in time to pay less for it. It must already be on the stack to be a valid target for Truss’s ability.

Indefinite articles such as “a” in “draw a card” aren’t number words and can’t be modified.

You can’t use Truss’s last ability to change the 1 in Truss’s last ability to a 0. Nice try though.

You can’t change any instance of X.

Note that you’re changing numbers, not digits. For example, if you remove two hack counters from Truss, you could change 10 to 12 or 8. You couldn’t change it to 30.

You can change the number of dice rolled for an effect only if the original instruction included a number word to describe how many dice to roll. You can’t change the “a” in “roll a six-sided die,” but you could change the “two” in “roll two six-sided dice.”

Truss’s last ability affects a single number or number word on the spell or permanent. It can be anything—power, toughness, something in rules text, the name of a card (Four Wishes, anyone?), collector number, and so on. Mark says you can even change the copyright date, but don’t tell the lawyers.

You can change the number of sides on a die. If you don’t have any thirteen-sided dice handy, you can use other dice to simulate the rolls.