/**
   * Creates a new subscription object with the given virtual subscription and unsubscription time.
   *
   * @constructor
   * @param {Number} subscribe Virtual time at which the subscription occurred.
   * @param {Number} unsubscribe Virtual time at which the unsubscription occurred.
   */
  var Subscription = Rx.Subscription = function (start, end) {
    this.subscribe = start;
    this.unsubscribe = end || Number.MAX_VALUE;
  };

  /**
   * Checks whether the given subscription is equal to the current instance.
   * @param other Subscription object to check for equality.
   * @returns {Boolean} true if both objects are equal; false otherwise.
   */
  Subscription.prototype.equals = function (other) {
    return this.subscribe === other.subscribe && this.unsubscribe === other.unsubscribe;
  };

  /**
   * Returns a string representation of the current Subscription value.
   * @returns {String} String representation of the current Subscription value.
   */
  Subscription.prototype.toString = function () {
    return '(' + this.subscribe + ', ' + (this.unsubscribe === Number.MAX_VALUE ? 'Infinite' : this.unsubscribe) + ')';
  };