function reduce(collection, iteratee, accumulator) {
      var func = isArray(collection) ? arrayReduce : baseReduce,
          initAccum = arguments.length < 3;

      return func(collection, getIteratee(iteratee, 4), accumulator, initAccum, baseEach);
    }
Reduces `collection` to a value which is the accumulated result of running
each element in `collection` thru `iteratee`, where each successive
invocation is supplied the return value of the previous. If `accumulator`
is not given, the first element of `collection` is used as the initial
value. The iteratee is invoked with four arguments:
(accumulator, value, index|key, collection).

Many lodash methods are guarded to work as iteratees for methods like
`_.reduce`, `_.reduceRight`, and `_.transform`.

The guarded methods are:
`assign`, `defaults`, `defaultsDeep`, `includes`, `merge`, `orderBy`,
and `sortBy`

@static
@memberOf _
@since 0.1.0
@category Collection
@param {Array|Object} collection The collection to iterate over.
@param {Function} [iteratee=_.identity] The function invoked per iteration.
@param {*} [accumulator] The initial value.
@returns {*} Returns the accumulated value.
@see _.reduceRight
@example

_.reduce([1, 2], function(sum, n) {
  return sum + n;
}, 0);
// => 3

_.reduce({ 'a': 1, 'b': 2, 'c': 1 }, function(result, value, key) {
  (result[value] || (result[value] = [])).push(key);
  return result;
}, {});
// => { '1': ['a', 'c'], '2': ['b'] } (iteration order is not guaranteed)