defining a print method

In the following session we define an xx class and a print method
for it.  If I invoke it with print then it prints as expected but if
we just type xx at the R prompt then we get nothing back.  How
do we define the print method so that it works in the latter
case too?

xx <- structure("abc", class = "xx")
print.xx <- function(x, ...) shQuote(x)

print(xx) # ok

[1] "\"abc\""

xx # no output ????

sessionInfo()

R version 2.2.1, 2005-12-20, i386-pc-mingw32

attached base packages:
[1] "methods"   "stats"     "graphics"  "grDevices" "utils"     "datasets"
[7] "base"