Wyo. Stat. § 15-8-104: Order of redemption.

If serial bonds are issued the city or town shall redeem them as they become due. If redeemable bonds are issued the city or town, during the last half of the term the bonds have to run, shall redeem annually a portion of the principal equal to a sum produced by taking the whole amount of bonds outstanding and dividing it by the number of years the bonds then have to run.