Wis. Stat. § 229.49: Bond security.

A district may secure bonds by a trust agreement, trust indenture, indenture of mortgage or deed of trust by and between the district and one or more corporate trustees. A bond resolution providing for the issuance of bonds so secured may mortgage, pledge, assign or grant security interests in some or all of the revenues and property of the district issuing the bonds and may contain those provisions for protecting and enforcing the rights and remedies of the bondholders that are reasonable and proper and not in violation of law. A bond resolution may contain other provisions determined by the board of directors to be reasonable and proper for the security of the bondholders.