Modifying the counter variable within a FOR loop is forbidden
for nCtr = nStart to nEnd step nInc
nCtr = nCtr div 2 // NO !!!
The reason behind this restriction is that allowing the counter variable to change could lead to an infinite loop (as would happen in the above example). If you feel you must absolutely change the value of the counter variable, this is a sign that you should be using a while loop instead of a for loop. For a discussion on which of the loops to use, see Looping Instructions.