Dear all, This is a stub for trial "package": function void mymatr(int x, matrix ma[null]) if isnull(ma) txt = "null matrix" print txt else print ma endif end function The packages editor GUI dislikes [null]: After saving .gfn in "Edit function code" window I have: function void mymatr (int x, matrix ma) if isnull(ma) txt = "null matrix" print txt else print ma endif end function [null] vanished Oleh