A variable-arity function from quantities to quantities (q.v.). When applied to a number or other numeric value (pair of numeric-intervals), percentage returns the instance of interval that is its argument (the interval between its two arguments) divided by 100. For example (percentage 110) is 1.1, and (percentage 10 20) is (unity 0.1 0.2): the interval between 0.1 and 0.2 inclusive.