Requires

Lithoform Engine must be on the battlefield. Corridor Monitor must be in hand. Mana Echoes must be on the battlefield. Mana needed:

Steps

1. Cast Corridor Monitor for .
2. While Corridor Monitor is on the stack copy it with Lithoform Engine.
3. Let the token copy resolve.
4. Arrange the triggers for Corridor Monitor and Mana Echoes so Corridor Monitor untaps Lithoform Engine first.
5. Before Mana Echoes resolves copy the original Corridor Monitor with Lithoform engine.
6. Corridor Monitor untaps Lithoform Engine and Mana Echoes adds .
7. Copy the first Corridor Monitor while its still on the stack.
8. The 3rd Corridor Monitor token untaps Lithoform Engine and adds .
9. Copy Corridor Monitor with Lithoform Engine one more time to net and untapping Lithoform Engine.
10. Repeat for infinite Corridor Monitors and colorless mana.

Results