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

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

Result:
  - Infinite landfall triggers
  - Infinite red mana
  - 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
  - Ancient Den must be in the graveyard
  - Great Furnace must be on the battlefield
  - Goblin Welder does not have summoning sickness.
[END OF COMBO PREREQUISITES]
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