Dude, seriously? This is a programming group, not a group for requesting free lisp routines. IMO, if you post here, you should be a programmer or learning to become one. Your response indicates that you're neither learned nor interested in investing any effort in learning, which is rude and insulting to the rest of the members. Lee Mac wrote a custom lisp routine to your specification for free. He probably skipped some other important activity so he could spend an hour or two to help you out. Now you can't be bothered to invest 10 minutes to read the code and do some internet research in order to learn how to use it? I think you owe Lee Mac an apology.
Though your comments in this thread appear to be unheeded, I would like to thank you for your support Owen, I do appreciate it. Perhaps I am sending the wrong message by supplying code for the program in its entirely, albeit fully commented with the intention that the OP can learn from my post, rather than blindly copy/pasting the code, which evidently appears to be the case - judging from the subsequent responses from the OP, I might have better invested the time spent annotating the code elsewhere. I think in future I shall instead provide hints to demonstrate how a task may be accomplished, or rather ask the OP to post their attempt at the problem before providing any full solution, if at all. I do enjoy programming, and like to share the programs that I write 'for fun', but maybe this generosity is sending the wrong message to the community leading to members requesting free code.