How can you create a combo with Goblin Welder, Retreat to Coralhelm, Darksteel Citadel, Ancient Den?
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[START OF COMBO]This combo can be formed with Goblin Welder, Retreat to Coralhelm, Darksteel Citadel, Ancient Den

Color identity: URW
Mana cost: 
Steps:
  1. Activate Goblin Welder by tapping it, causing you to sacrifice Ancient Den and return Darksteel Citadel from your graveyard to the battlefield.
  2. Darksteel Citadel enters the battlefield, triggering Retreat to Coralhelm, untapping Goblin Welder.
  3. Activate Darksteel Citadel by tapping it, adding {C}.
  4. Activate Goblin Welder by tapping it, causing you to sacrifice Darksteel Citadel and return Ancient Den from your graveyard to the battlefield.
  5. Ancient Den enters the battlefield, triggering Retreat to Coralhelm, untapping Goblin Welder.
  6. Activate Ancient Den by tapping it, adding {W}.
  7. Repeat.[END OF COMBO STEPS]

Result:
  - Infinite colorless mana
  - Infinite landfall triggers
  - Infinite white mana[END OF COMBO RESULT][END OF COMBO]

Other prerequisites:
  - Goblin Welder must be on the battlefield
  - Retreat to Coralhelm must be on the battlefield
  - Darksteel Citadel must be in the graveyard
  - Ancient Den must be on the battlefield
  - Goblin Welder does not have summoning sickness.
[END OF COMBO PREREQUISITES]
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