Urza's Sylex

{2}{W}{W}, {T}, Exile Urza's Sylex: Each player chooses six lands they control. Destroy all other permanents. Activate only as a sorcery.
When Urza's Sylex is put into exile from the battlefield, you may pay {2}. If you do, search your library for a planeswalker card, reveal it, put it into your hand, then shuffle.

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The triggered ability of Urza's Sylex will resolve before the activated ability. This means that you choose whether to pay {2} and search your library before any player chooses lands. Notably, this means that the other players will get to know which planeswalker card you searched for before choosing any lands.