// Reduces an array or object to a single value by repetitively calling
  // iterator(accumulator, item) for each item. accumulator should be
  // the return value of the previous iterator call.
  //
  // You can pass in a starting value for the accumulator as the third argument
  // to reduce. If no starting value is passed, the first element is used as
  // the accumulator, and is never passed to the iterator. In other words, in
  // the case where a starting value is not passed, the iterator is not invoked
  // until the second element, with the first element as its second argument.
  //
  // Example:
  //   var numbers = [1,2,3];
  //   var sum = _.reduce(numbers, function(total, number){
  //     return total + number;
  //   }, 0); // should be 6
  //
  //   var identity = _.reduce([5], function(total, number){
  //     return total + number * number;
  //   }); // should be 5, regardless of the iterator function passed in
  //          No accumulator is given so the first element is used.


  _.reduce = function(collection, iterator, accumulator) {

  };