[R] For multiple trellis plot, how to add a global title

[alexfang@stat.rutgers.edu]

I use print.trellis() to draw multilple plots in a single frame, but than how 
could add a global title and subtitle to the final plot.  Thank you!

eg code:
print(position=(0,0,1,0,5), more=T, xyplot(...))
print(position=(0,0.5,1,0,1), more=F, xyplot(...))

##### Will produce error if I put the following:
# title(main="global main title")
# title(sub="global sub title")



What should I do?  thank you!

best, Alex