function descending(a, b) {
  return a < b ? 1 : a > b ? -1 : 0;
}
Compare function sorting arrays in descending order.  Safe to use for numeric values.
@param {*} a The first object to be compared.
@param {*} b The second object to be compared.
@return {number} A negative number, zero, or a positive number as the first
    argument is greater than, equal to, or less than the second.