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