I'm not sure but I think it's a bug. I tried to use the while function and when I want to converge a number to a constant there is no problem. But when I want to converge two numbers to each other like below, the program stops working.

Example:

x:=0
a:=20-2*x
b:=15-x
while a>b x:=x+1

This is not a bug.
Second line (a:=20-2*x) program calculate as a:=20, becouse you've define x:=0; third line has similar result, so your condition in while loop is allways true.

Here is solution:

(download an example)