[Xy-pic] 2 cell feature

[ruggero pagnan]

Dear all,
I need some help in using the 2-cell feature of xy-pic.
I already referred to the xy-pic reference manual but I still have problems.
Simply, I would like to be able to use such commands as \rtwocell and others 
in order to draw so called "pasting diagrams". 
During typesetting, my laptop doesn't give me any error message but the 
diagrams don't appear in the dvi file.
I use \usepackage[all]{xy}

Do I need something else?

Thanks a lot for your help,

R. Pagnan