If you know how to program using Xlisp-Stat, you can extend Arc to add new smoothers, new fitting methods, new menu items, and the like. The document extend.pdf explains how to do the extension. You will almost certainly also need some additional computer code to see how Arc is written. For this purpose, you can get the directory ExtendArc for Windows (zip file), Macintosh (sit file), or Unix (shar file).