Emil amberboy and others invested in oil and gas partnerships formed by vanguard group international, inc. each investor made a down payment in cash and signed a promissory note payable to the partnership for the balance of the investment. each note stated that its interest rate was to be determined by reference to a certain bank’s published prime rate. several months later, vanguard sold the notes to société de banque privée. suspecting that the investments were being handled fraudulently, amberboy and the other investors stopped making payments on the notes and filed a lawsuit against société de banque privée and others. one of the issues before the court was whether the notes were negotiable. the plaintiffs contended that because the interest rate on the notes could be calculated only by reference to a source outside the notes, the notes could not be negotiable instruments. how should the court rule?