@Barry Kelly
Although I understand your statement, but I really meant what I have written previously, and that is,
if a GDL programmer or an ArchiCAD user can choose the Pens of 0 and -1 with a Pen Parameter, then the same user should be able to choose those same pens in a Pen Array Parameter
The application of the case (long story short), I have made an intricate GDL object, were I have put all my pens' numbers in a Pen Parameter Array, were the application of the specific pen in the array is dependent on previous selections made by the user, but as Pen Array Parameter can't save the value of -1, I had to revise every line of the script to make sure it wouldn't cause the inconvenience (Master Script alone 3,000 lines approx).
I know that it may appear more complicated than it really is, but the benefit is that when the firm's users want to change those pens knowing an absolute minimum of GDL (below a novice level), s/he can change them quickly and efficiently from one place in the code, without messing the rest of the script, and with comments that guide his/her choices.
I can't say more than that because of the NDA that I have signed with the firm.
@Peter Baksa
Thank you very much, and thank you again for the advice, lucky it only affected one portion of the script that was easy to fix for the time being.
A good friend of mine have once told me that I´m so brute that I´m capable of creating a GDL script capable of creating GDLs.