Given an Existing plot object is it possible to add a layer **UNDERNEATH** an existing layer? 

Example, in the graph below, is it possible to add `geom_boxplot()` to `P` such that the boxplot appears **underneath** `geom_point()`?

    ## Starting from: 
    library(ggplot2)
    P <- ggplot(data=dat, aes(x=id, y=val)) + geom_point()

P + ggtitle("Starting From Here") + xlab("") + ylab("")
ggsave("~/P_initial.png", width=4.5, height=4.5)

    ## This adds boxplot, but obscures some of the points
    P + geom_boxplot() 

last_plot() + ggtitle("Incorrect Output") + xlab("") + ylab("")
ggsave("~/P_wrong.png", width=4.5, height=4.5)

### Expected Output: 

    # Which is essentially
    ggplot(data=dat, aes(x=id, y=val)) + geom_boxplot() + geom_point() 

last_plot() + ggtitle("Expected Output") + xlab("") + ylab("")
ggsave("~/P_right.png", width=4.5, height=4.5)


However, this involves re-coding all of P after the point insertion of the new layer.

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Bonus question:  If there are multiple layers in the existing plot, is it possible to indicate where specifically to insert the new layer (with respect to the existing layers)?

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### SAMPLE DATA 

    set.seed(1)
    N <- 100
    id <- c("A", "B")
    dat <- data.frame(id=sample(id, N, TRUE), val=rnorm(N))

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NOTE TO SELF:  This has since been updated.  See utilsRS.

answer: 

    insertLayer <- function(P, newLayer, at=1) {
    ## at: the index to the list wher newLayer will be inserted.
    ##     if  at == length(P$layers), then it is inserted one below the last layer
    ##     if  at  > length(P$layers), then it is inserted as the top layer
    ##         (though at that point, just use `+`)

      lis  <- P$layers
      leng <- length(lis)
      # if strictly greater, insert at end

      if (at < 0)
        at <- (leng+at)+1

      if (at == 0)
        stop("`at` cannot be 0. Did you mean `at=1`?")

      P$layers <- {
          if (at > leng)
            c(lis,newLayer)
          else if(at == 1)
            c(newLayer,lis)
          else 
            c(lis[1:at-1], newLayer, lis[at:leng])
       }

      return(P)
    }

lis <- setNames(LETTERS[1:5], 1:5)
lis

    1     2     3     4     5 
   "A"   "B"   "C"   "D"   "E" 
-5    -4    -3    -2    -1