Newer readers may wonder what these mysterious numbers are that we put at the end of the lines. These relate to an ingenious checking program sent in by reader Adrian Wilkins to help you check that you have typed in your listing correctly. And for anyone wanting to take advantage of this invaluable service we are reprinting the listing.

What this program does is look at your listing and print out a code at the end of each line. Check this code with the code in the magazine and if they differ look closely at that line for your mistake.

Of course you have to get this program running without the benefit of this high technology. But once you have it running successfully you can use it to check any listing. The only thing you have to remember is to save your listing in ASCII form: where you would normally type SAVE "PROG" to save it, type SAVE "PROG",A, a instead.

Then run the Checker program. When it asks for the filename enter the name of your listing saved in ASCII form. It then prints out the listing with the numbers. Before you can make any changes to the listing you will have to load it in the normal way - LOAD "FILENAME" and make your changes.