we were arguing, whether a tarmogoyf targeted by merfolk trickster is 0/0 or 0/1.
i initially said it's 0/0 and dies (which is the way magic online apperently does it in the moment), because it loses it's CDA completely, so the whole “~'s power is equal to the number of card types among cards in all graveyards and its toughness is equal to that number plus 1” is gone, leaving a creature without defined p/t, hence 0/0.

NOTES AND RULES INFORMATION FOR MERFOLK TRICKSTER say otherwise:
“If the target creature has power and toughness written as */* with an ability that defines its power and toughness, it’s 0/0 when it loses all abilities. If its power and toughness are written as */*+1, it’s 0/1, and so on.
(2018-04-27)”
(note that tarmogoyf is written */1+*, not */*+1 but that shouldnt make a difference, right?)

so it appears to me the CDA is only the “…is equal to that number..” part, without the “plus 1”

while i accept that, i'd also like to understand it, i can't find the rules from which to conclude this outcome,
who can tell me the exact relevant rule here?

what is that “1+”, if it's not part of the CDA? a modifier?

Edited Bertil Angermann (April 27, 2018 03:36:07 PM)

Tarmogoyf has a characteristic-defining ability that defines its power and toughness. (“Tarmogoyf's power is equal to the number of card types among cards in all graveyards and its toughness is equal to that number plus 1.”) It also has a printed power and toughness. (* /1+*)

Normally, Tarmogoyf's CDA overrides its printed power and toughness. However, if Merfolk Trickster has removed that ability, we are left to determine Tarmogoyf's power and toughness the same way we determine those values for any other creature: by looking at what's printed in its power/toughness box.

For Tarmogoyf, what's printed there is “* / 1+*”. Since * does not have a numerical value, we use 0 instead, just as we do for any other number that can't be determined. Therefore, Tarmogoyf's power and toughness is “0 / 1+0”, also known as 0/1.

So why is Tarmogoyf a 0/1 under the effects of a Merfolk Trickster? For the same reason a Dauntless Bodyguard would be a 2/1, or a Pardic Wanderer would be a 5/5: that's what's printed on the card.