How do you generate a prediction interval from a regression tree that is fit using rpart? 

It is my understanding that a regression tree models the response conditional on the mean of the leaf nodes. I don't know how to get the variance for a leaf node from the model, but what I would like to do is simulate using the mean and variance for a leaf node to obtain a prediction interval.

[Predict.rpart()](http://cran.r-project.org/web/packages/rpart/vignettes/longintro.pdf) doesn't give an option for interval. 

**Example:** I fit a tree with iris data, but predict doesn't have an option, "interval"

```
> r1 <- rpart(Sepal.Length ~ ., cp = 0.001, data = iris[1:nrow(iris)-1,])
> predict(r1,newdata=iris[nrow(iris),],type = "interval")
Error in match.arg(type) : 
  'arg' should be one of “vector”, “prob”, “class”, “matrix”

```