This risk-free arbitrage trade locked in a small profit for the institutional investor and kept the SuperUnit market price in line with its underlying value.

FORBES: ETFs Turn 20-Years Old in the U.S.

However, one of this summer's best investment tips, currency arbitrage, is risk-free.

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But unlike closed-end funds, if SuperUnits developed a discount to its underlying securities value, then institutional investors could arbitrage the units and profit risk-free.

FORBES: ETFs Turn 20-Years Old in the U.S.