You control at least one additional creature with mana value 2 or less that can be cast using colored mana.
({U} magic symbol) ({B} magic symbol) available.
Steps
Cast Cavern Harpy by paying ({U} magic symbol) ({B} magic symbol) .
Oltec Matterweaver triggers twice, creating two copies of a Treasure token you control.
When Cavern Harpy enters the battlefield, it triggers twice, returning itself and another creature with mana value 2 or less that can be cast using colored mana from the battlefield to your hand.
If needed, activate a Treasure by tapping and sacrificing it, adding one mana of any color.
Repeat step 4 one additional time if you do not have enough mana to cast the other creature returned to your hand in step 3.
Cast the other creature returned to your hand in step 3 by paying its mana cost.
Oltec Matterweaver triggers twice, creating two copies of a Treasure token you control.
Activate two Treasures by tapping and sacrificing them, adding ({U} magic symbol) ({B} magic symbol) .
Repeat.
Notes
If the other creature has mana value 1 or less, this combo nets infinite Treasure tokens, and you may either make Gnome tokens or copy other artifact tokens using Oltec Matterweaver's triggers.