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Date: Sun, 14 Jan 2018 17:18:13 -0800 (PST)
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Subject: ADA.STRINGS.INDEX_ERROR : a-strunb.adb:782
From: Mehdi Saada <00120260a@gmail.com>
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Xref: reader02.eternal-september.org comp.lang.ada:49900
Date: 2018-01-14T17:18:13-08:00
List-Id: <comp.lang.ada>

This if statement exactly, seems to raise
"ADA.STRINGS.INDEX_ERROR : a-strunb.adb:782"(No indication of line, so I can't know more).
What does this exception means ? You can see there's no string type variable to be accessed, here.
CHAINE is of UNBOUNDED_STRING type. LA_PILE is a stack of characters, EMPILER means "putting on the stack's top" in french.

if IS_BASIC(ELEMENT(CHAINE,IND)) then
   EMPILER(LA_PILE,ELEMENT(CHAINE,IND));
else DELETE(CHAINE, IND,IND);
end if;

It's enclosed in:
for Ind in 1..Length(CHAINE) loop
        begin
            if IS_BASIC(ELEMENT(CHAINE,IND)) then
                    EMPILER(LA_PILE,ELEMENT(CHAINE,IND));
            else DELETE(CHAINE, IND,IND);
            end if;
         end;
end loop;

Thanks