function Plan(expression, selector) {
      this.expression = expression;
      this.selector = selector;
  }

  Plan.prototype.activate = function (externalSubscriptions, observer, deactivate) {
    var self = this;
    var joinObservers = [];
    for (var i = 0, len = this.expression.patterns.length; i < len; i++) {
      joinObservers.push(planCreateObserver(externalSubscriptions, this.expression.patterns[i], observer.onError.bind(observer)));
    }
    var activePlan = new ActivePlan(joinObservers, function () {
      var result;
      try {
        result = self.selector.apply(self, arguments);
      } catch (e) {
        observer.onError(e);
        return;
      }
      observer.onNext(result);
    }, function () {
      for (var j = 0, jlen = joinObservers.length; j < jlen; j++) {
        joinObservers[j].removeActivePlan(activePlan);
      }
      deactivate(activePlan);
    });
    for (i = 0, len = joinObservers.length; i < len; i++) {
      joinObservers[i].addActivePlan(activePlan);
    }
    return activePlan;
  };

  function planCreateObserver(externalSubscriptions, observable, onError) {
    var entry = externalSubscriptions.get(observable);
    if (!entry) {
      var observer = new JoinObserver(observable, onError);
      externalSubscriptions.set(observable, observer);
      return observer;
    }
    return entry;
  }