double-sided printing

[hendrik@topoi.pooq.com]

I use CUPS's xpp.  It lets me tell it whether to print double-sided.
For files from some programs, it does double-sided when I tell xpp to do 
so.  For postscript files from my own programs, it prints 
single-sided anyway.  What kind of postscript code do I need to use so 
for end-of-page so that xpp and my printer will deign to allow 
double-sided printing? 

My printer is a brother HL-1870N networked Postscript printer.

-- hendrik