Present Arms swaps your library with another deck you own from outside the game and use that one instead ("library" and "deck" both refer to the same thing, but the former is used to refer to it in-game, whereas the latter is used to refer to it outside of the game)
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That said, while the card is a good catch-all for red decks, its not an expensive reprint
I swing in, going to 14 against his 13, cast Portent on my opponent, seeing Stifle, Island, and Impulse
Make sure your main deck contains exactly the number of cards required for your format (typically 100 for Commander, while Standard, Modern, and Pioneer have 60)
