I recently bought Dominion: Cornucopia and we played our first game yesterday. Something that confused me was the Horn of Plenty card which has a treasure value equal to the number of different card titles you have in play at that moment.
From the rule book I understood that if you'd first use other treasure cards to buy a card from the stock and then play Horn of Plenty(assuming you'd have more than one buy action), Horn of Plenty would take into account the treasure cards.
But in case I'd have only one buy option, am I allowed to play Horn of Plenty together with other treasure cards and let them count towards the total of Horn of Plenty?