Hello,
I think the function 'FORMAT' does what you tried with FACSUM.
I tried FORMAT(s * 'diff(z,s) + z * 'diff(z,s) + 1, %poly('diff(z,s)))
and got (z+s)*'diff(z,s,1)+1.
The function comes in a package, which can be downloaded here:
http://math.nist.gov/~BMiller/computer-algebra/
(Installation does not work the way it should on my machine, I had to
'compile_file' the format.lisp and coeflist.lisp files by hand and
'loadfile' the resulting .o files before using FORMAT.
The package also provides its own Poisson function, as far as I know.
Regards
Markus