function Promise(descriptor, fallback, inspect) {
    if (fallback === void 0) {
        fallback = function (op) {
            return reject(new Error(
                "Promise does not support operation: " + op
            ));
        };
    }
    if (inspect === void 0) {
        inspect = function () {
            return {state: "unknown"};
        };
    }

    var promise = object_create(Promise.prototype);

    promise.promiseDispatch = function (resolve, op, args) {
        var result;
        try {
            if (descriptor[op]) {
                result = descriptor[op].apply(promise, args);
            } else {
                result = fallback.call(promise, op, args);
            }
        } catch (exception) {
            result = reject(exception);
        }
        if (resolve) {
            resolve(result);
        }
    };

    promise.inspect = inspect;

    // XXX deprecated `valueOf` and `exception` support
    if (inspect) {
        var inspected = inspect();
        if (inspected.state === "rejected") {
            promise.exception = inspected.reason;
        }

        promise.valueOf = function () {
            var inspected = inspect();
            if (inspected.state === "pending" ||
                inspected.state === "rejected") {
                return promise;
            }
            return inspected.value;
        };
    }

    return promise;
}
Works almost like "finally", but not called for rejections.
Original resolution value is passed through callback unaffected.
Callback may return a promise that will be awaited for.
@param {Function} callback
@returns {Q.Promise}
@example
doSomething()
  .then(...)
  .tap(console.log)
  .then(...);