How can you create a combo with Timestream Navigator, Riptide Laboratory, Archmage of Echoes?
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[START OF COMBO]This combo can be formed with Timestream Navigator, Riptide Laboratory, Archmage of Echoes

Color identity: U
Mana cost: {1}{U} available the first turn, with {4}{U}{U}{U}{U} each following turn
Steps:
  1. Cast Timestream Navigator by paying {1}{U}.
  2. Archmage of Echoes triggers, copying the Timestream Navigator spell.
  3. Resolve both Timestream Navigator spells.
  4. If you don't have the city's blessing, either the Timestream Navigator token or the nontoken Timestream Navigator gives you the city's blessing.
  5. During your next turn, activate Riptide Laboratory's second ability by paying {1}{U} and tapping it, returning the nontoken Timestream Navigator to your hand from the battlefield.
  6. Activate the ability of the Timestream Navigator token by paying {2}{U}{U}, tapping it and putting on the bottom of your library, causing you to take an extra turn after that one.
  7. Repeat.[END OF COMBO STEPS]

Result:
  - Infinite turns[END OF COMBO RESULT][END OF COMBO]

Other prerequisites:
  - Timestream Navigator must be in your hand
  - Riptide Laboratory must be on the battlefield
  - Archmage of Echoes must be on the battlefield
  - You have the city's blessing, and/or control (at least) eight permanents
[END OF COMBO PREREQUISITES]
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