You want to buy in the store Union Pacific steel lined box the cost of which 175 bonds / nuggets, but you have to account 100 bonds and 75 nuggets . You have two outputs: 1. Earn 175 bond. 2. Earn 175 nuggets. I propose to introduce nuggets exchange for the bonds at a rate of 1:1 (or 1:3 or 1:5, etc.) Exchange of bonds for the nuggets are not implemented.