let initialize_pspicture () =
if (!inside_pspicture) then failwith "Double pspicture not allowed"
else begin
inside_pspicture := true;
let (r,d,l,u) = !window in
ext (Printf.sprintf
"{\\begin{pspicture}(%.3f,%.3f)(%.3f,%.3f)\n" r d l u);
ext (Printf.sprintf "\t\\psset{unit=%.3f}\n" (!unit_factor));
end