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You may look at the top card of your library any time.
Would You Like A...? — Once each turn, you may play a historic land or cast a historic spell from the top of your library. When you do, create a Food token. (Artifacts, legendaries, and Sagas are historic.)
Essentially, The Fourth Doctor lets you play with the top card of your library revealed only to you. Knowing what that card is becomes part of the information you have access to, just like you can look at the cards in your hand. You may look at the top card of your library whenever you want, even if you don't have priority. This action doesn't use the stack.
If an effect instructs you to choose a creature type, you may choose Time Lord.
If the top card of your library changes during the process of casting a spell or activating an ability, you can't look at the new top card until the process of casting the spell or activating the ability ends (all targets are chosen, all costs are paid, and so on).
Neither "Time" nor "Lord" are creature types. Some older cards were printed with the subtype "Lord," but all of those cards have updated Oracle card text that removed that type.
Unlike other creature types in Magic that are each only one word, the two words "Time Lord" represent a single creature subtype. Time Lord is the only two-word creature type.
