The fund, using a passive or indexing investment approach, seeks investment results that correspond generally to the performance, before fees and expenses, of the Solactive Municipal Infrastructure Revenue Bond Index (the Underlying Index), which is designed to track the performance of the US long-term tax exempt bond market, consisting of infrastructure revenue bonds. The fund uses a representative sampling indexing strategy in seeking to track the Underlying Index, meaning that it will generally invest in a sample of securities in the index whose risk, return and other characteristics resemble the risk, return and other characteristics of the Underlying Index as a whole. The fund will invest at least 80 percent of its total assets (but typically far more) in instruments that comprise the Underlying Index.