Prolog Problem with "is" function

Question

I am having a problem with prolog that I do not understand. I have used the tracer to follow the problem, and this is what happens

....
(8) 8 NEXT  value(debt, p9, Orly)
  (8) 8 *EXIT  value(debt, p9, low)
  (9) 8 CALL  P is low
S (10) 9 CALL  error_handler(21, P is low, eclipse, sepia_kernel)
...error crap, more "leaves" calls....

What is wrong with the call, "P is low"?

Answer 1

low is an atom, not a mathematical expression that is could evaluate. Maybe you want to write it upper case, i.e. Low, so that it is a variable?