The fund normally invests at least 80% of its net assets plus any borrowings for investment purposes, in municipal bonds issued by or on behalf of the states, territories and possessions of the United States, the District of Columbia and their respective authorities, agencies, instrumentalities and political subdivisions, the income from which, in the opinion of bond counsel, is exempt from regular federal income and federal alternative minimum taxes. It may invest up to 10% of its total assets in such unrated securities.