The current active edition of this book is: Fisrt Edition 2013 . Please check the folio / copyright page at the start of your book to ascertain which edition you have.
Program 23a has an incorrect REM statement at line 10 where it suggests it is Program 21a. Line 10 should read:
10 REM >Prog23a
This is what happens when you decide to rearrange chapters at the last minute!
Last updated: 8th December 2013