Rebol Programming/rebol-link?

It's set by rebol to true if are you using Rebol/Link; otherwise it is set to false.

Rebol/Link is a client for Rebol IOS for group communications, for more information look a these pages:
 * http://www.rebol.com/ios/ios-intro.html
 * http://www.rebol.com/ios/ios-faq.html