code: function(expression, onAdd, onRemove, callbackContext) {
      if (deepDelimiter.test(expression)) {
        return this.trackDeeply(expression, onAdd, onRemove, callbackContext);
      }
      var observer = this._observations.createMemberObserver(expression, onAdd, onRemove, callbackContext || this);
      this._observers.push(observer);
      if (this.observersEnabled) observer.bind(this._context);
      return observer;
    }
docs: Observe an expression and call `onAdd` and `onRemove` whenever a member is added/removed from the array or object.
  @param {String} expression The expression to observe
  @param {Function} onAdd The function which will be called when a member is added to the source
  @param {Function} onRemove The function which will be called when a member is removed from the source
  @return {Observer} The observer created