Support for Assemble
Used in the Unstable casual variant, also appears on the traditional Steamflogger Boss card.
If you have a Contraption deck, and if there are cards in it, the top card is revealed and you choose a sprocket to assemble the Contraption on. The Contraption is put onto the battlefield under that sprocket.
Assembling is done by a player or a card, usually a Rigger creature.