It is hereby declared that it is a public purpose for a local agency to issue refunding bonds for the purposes set forth in this article and to invest and reinvest the proceeds thereof, and any other funds legally available therefor, for the purposes set forth herein; provided, however, that it is the intent of this article, and this article shall be so construed, that in no single fiscal year shall a tax be levied or shall funds of a local agency other than those expressly permitted herein be used to pay the principal of and interest and redemption premium, if any, on both the refunding bonds and on the bonds to be refunded.