Schrödinger macskája

Each player selects one card from their hand and places it face down; those face-down cards are not used in the current round (page 4).

After placing the face-down card, each player in clockwise order (starting with the start player) places a token on the number on their player board that represents how many tricks they predict they will win (page 4).

Two rules must be met: the research-board space for that color and card number must be empty, and you must still have your player token on the X of that color on your player board; additionally the start player may not declare red unless a token already exists on the red row or they cannot declare any other color (pp.5-6, 12).

Yes — when you declare a color other than the led color, you remove the player token on your player board that was on the X of the led color (if it hasn't already been removed) and you can no longer declare that led color for the rest of the round (page 6).

If any player declared a color different from the led color, resolve color strength first: red (trump) beats all, otherwise only cards of the led color remain; then among the remaining cards the highest number wins the trick (page 7).

A paradox occurs when a player cannot legally declare any color for any card in their hand; when it happens you immediately stop the trick phase, the player reveals their hand, no one wins that trick, tokens placed during the interrupted trick remain, and the paradox-causing player scores −1 point per trick they won that round and forfeits any bonus (pp.8-9).