Wis. Stat. § 62.621
At the time of issuance or in anticipation of the issuance of appropriation bonds under s. 62.62, or general obligation promissory notes under s. 67.12(12), to pay unfunded prior service liability with respect to an employee retirement system, or at any time thereafter so long as the appropriation bonds or general obligation promissory notes are outstanding, a 1st class city may enter into agreements or ancillary arrangements relating to the appropriation bonds or general obligation promissory notes, including trust indentures, liquidity facilities, remarketing or dealer agreements, letters of credit, insurance policies, guaranty agreements, reimbursement agreements, indexing agreements, and interest exchange agreements. Any payments made or amounts received with respect to any such agreement or ancillary arrangement shall be made from or deposited as provided in the agreement or ancillary arrangement.
Wis. Stat. § 62.621