code: function addProperty(propertyName, defaultValue){
  
      if(model[propertyName]){
        throw new Error("The property \"" + propertyName + "\" is already defined.");
      }
  
      var property = ReactiveProperty(defaultValue);
      property.propertyName = propertyName;
      model[propertyName] = property;
      lastPropertyAdded = propertyName;
      return model;
    }
docs: Adds a property to the model that is not exposed, meaning that it is not included in the configuration object.