Contour plots on xz and yz surfaces

["K. Bowman" <k-bowman(at)null.tamu.edu>]

I'm looking for something like the library routine PLOT_3DBOX to draw
multiple contour plots on the inside surfaces of a box.  In fact, I
would like to overplot the contour plots on a PLOT_3DBOX plot.

CONTOUR has a ZVALUE keyword that allows one to position an "xy"
contour plot in the z-dimension, but AFAIK, there is no similar
"XVALUE" or "YVALUE" keyword.

Can anyone provide an example of how to draw a contour plot on an "xz"
surface in a 3-D plot?

Thanks, Ken