_requesterDecrementingPromise (dfd) {
		// regardless of whether we're resolved with a 'res' or 'err',
		// we want to decrement requests. the appropriate 'success' or 'error'
		// callback will be executed on whatever is chained after this method
		return this._parsePromise(dfd).fin( resOrErr => {
			this.decrementRequesters();
			return resOrErr;
		});
	}
Chain a promise with another promise that decrements
the number of expected requesters.