Overlaying trellis xyplot on contourplot

Friends: I wish to overlay data points on a contour graph.  The following
example produces a nice contour plot, but I have not mastered the concept
of using panel functions to modify plots. Can someone show me how to
overlay the data points (given after contour plot statement) on the
contourplot?
--Seth


model <- function(a,b,c,X1,X2)                        # provide model
function for contour plot
 {(exp(a + b*X1 + c*X2)) / (1 + exp(a + b*X1 + c*X2))}

g <- expand.grid(X1 = seq(0.40, 0.8,0.01), X2 = seq(0.03,0.99,0.03)) #
create gridded data for contour plot
a <- -37.61                               # Assign value to 'a' parameter
b <- 34.88                                # Assign value to 'b' parameter
c <- 28.44                                # Assign value to 'c' parameter
g$z<- model(a, b, c, g$X1,g$X2)                       # Create variable z
using gridded data, model, and variables

contourplot(z ~ X1 * X2,                              # specify the basic
equation for the contour plot
 data=g,                                  # Specify the data frame to be
used
 contour=TRUE,                                  # Make sure it adds
contours
 xlim=c(0.4,0.8), ylim=c(0.401,0.999), zlim=c(0,1),         # Set axis
ranges
 xlab="p(H)", ylab="p(H|H)",                    # Add axis labels
 region = TRUE,                                 # Add nice colors
 cuts=10                                  # Specify number of contour
slices
 )

# Data to superimpose as xyplot on the contourplot....

ph <-c(0.42,0.47,0.59,0.40)                     # Create a vector of values
under variable 'ph'
phh <-c(0.76,0.81,0.82,0.71)                          # Create vector of
values for variable 'phh'
d <- data.frame(ph,phh)                               # Group variables ph
& phh in data frame 'd'




Dr. Seth  W. Bigelow
Biologist, Sierra Nevada Research Center
Pacific Southwest Research Station, USDA Forest Service
Mailing address: 2121 2nd St Suite A101, Davis CA 95616
www.fs.fed.us/psw/programs/snrc/staff/bigelow
www.smbigelow.net
Phone: 530 759 1718
Fax: 530 747 0241