I need to show extended Ascii codes in GtkAda environment

[L Dries <bertus.dries@planet.nl>]

In a GtkAda environment I have the need to present some characters or 
strings with characters in the Extended character Ascii range for 
instance O with slash (216). I create a constant character with

Slash_null   : constant string(1 .. 1) := "" & character'Val(216);
and use it in some other part,
    symbols          : array (1 .. Max_Num, 1 .. 3) of string(1 .. 1) :=
      (("1", "A", "0"),
       ...,
       ("9", "I", "0"),
       (Slash_null, "J", "0"),
       ("A", "K", "0"),
       ...,
       etc.
The GPS program excepts these statements but in the output on the screen 
not only in a GtkAda window but alse in the list with a coming with a 
combo box it is presented as a rectangle with a cross inside. I can give 
another number in the "character'Val(216)" part but the result stays the 
same.
I suppose the problem is within GtkAda but I am not sure abot this.

I am using the highest level of Ada (2012).

How do I solve this problem?
-- 
L. Dries