The Biblioplex

{T}: Add {C}.
{2}, {T}: Look at the top card of your library. If it's an instant or sorcery card, you may reveal it and put it into your hand. If you don't put the card into your hand, you may put it into your graveyard. Activate only if you have exactly zero or seven cards in hand.

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Although you must have exactly zero or seven cards in your hand to activate the last ability, it doesn’t matter how many cards you have in your hand as that ability resolves.

If you don’t choose to either reveal the card or put it into your graveyard, the card stays on top of your library.