#Testing Counter unless pay {X} mechanic
#P1 casts Think Twice with {5} left in his mana pool and P2 counters with Syncopate using X = 3. P1 chooses to pay {3} and Think Twice resolves with {2} left in his mana pool.
[INIT]
firstmain
[PLAYER1]
hand:think twice
manapool:{1}{U}{5}
library:counterspell
[PLAYER2]
hand:syncopate
manapool:{3}{U}
[DO]
think twice
no
yes
syncopate
choice 3
think twice
syncopate
choice 0
[ASSERT]
firstmain
[PLAYER1]
hand:counterspell
graveyard:think twice
manapool:{2}
[PLAYER2]
graveyard:syncopate
[END]