code: function baseAggregator(collection, setter, iteratee, accumulator) {
        baseEach(collection, function(value, key, collection) {
          setter(accumulator, value, iteratee(value), collection);
        });
        return accumulator;
      }
docs: Aggregates elements of `collection` on `accumulator` with keys transformed
  by `iteratee` and values set by `setter`.
  
  @private
  @param {Array|Object} collection The collection to iterate over.
  @param {Function} setter The function to set `accumulator` values.
  @param {Function} iteratee The iteratee to transform keys.
  @param {Object} accumulator The initial aggregated object.
  @returns {Function} Returns `accumulator`.