All or Nothing

• A 12-slice pizza is on the table and everyone wants all of it. Each player holds four cards. On your turn, play one card into the centre and take one from the deck.
• Every round begins with a green card, or a black card played to add slices. Green cards add slices, red cards take slices away, black cards do either: you say which as you play.
• The pizza never has fewer than 0 or more than 12 slices.
• Whoever brings it to exactly 12 completes the pizza and scores one point for every card in the centre, so completing it early scores less.
• Double order: this round's pizza needs 24 slices. On the house: if you complete this round's pizza, your points are doubled; if someone else does, the coupon is wasted.
• After a round the cards in the centre are shuffled back into the deck; you keep your hand.
• Simple: hands are hidden. Challenge: hands are face up, so you can plan ahead, and a completed pizza scores double.

The printed rules leave some points open. This is how I read them here.

1. Who opens. The first seat opens the first round. After that, the player after whoever completed the last pizza opens. If nobody completed it, the player after the last opener opens.
2. No card to open with. A round opens with a green card or a black card added, as in the rules' first example, where a black ±5 opens the pizza ("5 + 2 = 7"). If the opener holds neither, the next player round the table who holds one opens, and play goes on from them. If nobody holds one, cards are turned from the deck until a green or black card appears. That card opens the pizza and the other turned cards go under the deck. The opener plays next.
3. What can be played. A number card can be played only if the pizza stays between 0 and 12 slices (0 and 24 after a Double order). A black card's direction is chosen when it is played, from the directions that fit.
4. No card fits. The player puts one card of their choice under the deck and takes the top card; that is their turn. If the deck is empty, they pass.
5. The deck runs out. Nobody draws, and play goes on from the hands. If every player passes in a row with the deck empty, the round ends with the pizza not completed. Nobody scores, and the round still counts as a round.
6. Double order and On the house are plays. Each goes into the centre and counts towards the score. Its player takes a card as usual; the slices do not change. Either can be played on any turn after the opening card. Double order makes 24 the target for the rest of that round only.
7. On the house. If the player who played it completes the pizza, they score twice the cards in the centre. If anyone else completes it, it counts as one ordinary card for them.
8. Scoring. The cards in the centre include the opening card, the card that completes the pizza and any Double order or On the house card.
9. Between rounds. The centre is shuffled back into the deck. Hands are kept; a hand left short because the deck ran out is filled back up to four.
10. The end. The game ends after the round in which someone reaches the points target, or after the chosen number of rounds, whichever was set at the table. The highest score wins; level scores share the win.
11. Challenge mode. The rules say playing face up "also means more points": in Challenge a completed pizza scores twice the cards in the centre. On the house doubles that again, to four times.
12. Speaking out. Every number card is said in the rules' form, including the opening card ("0 + 2 = 2").
13. Computer players go down this list. Complete the pizza if they can. If not, play On the house. If not, play Double order when the pizza is within 7 slices of 12. If not, play the card that leaves the others the fewest ways to complete it: in Simple they reckon from the cards they cannot see, in Challenge from the hands they can. Between cards that are equal on that, they prefer a count they could complete themselves next turn, then the larger pizza. With no card that fits, they put their highest card under the deck.