AS small alphanumeric signs

[tigger]

Hi Joe,
thank you very much for your answer.
I tried this page
http://h10032.www1.hp.com/ctg/Manual/c00554621.pdf
but I could not find the word "preemptive value".

Even with the find-function I did not succed to find something about "preemptive value".

Could you give me any link or reference where I could read something about it?

In the var key I can not find my variables 'a' , 'b' or 'c'.
Why did it work before with small letters on the stack? What dit I do (wrong), so that I get numbers for instead of letters?
I used the small letters for fomulars. I had a program "+". I put a and b on the stack. I got the answer "a+b" on the stack.