Our commentary on Attorney General Barr's decision not to prosecute James Comey (at least for now) on Espionage Act violations also included news about proof that two top Clinton aides --- Cheryl Mills and Heather Samuelson --- got highly irregular and extremely sweet immunity deals.  That news sparked this excellent reader inquiry from Phil:

QUESTION: Can immunity agreements be rescinded if it is found that they were issued improperly (as part of a conspiracy to cover up a crime)?

Okay, attorneys.  Since neither I nor my talented researchers are lawyers, we want to hear from you on this.  Under federal law, is an immunity agreement always sacrosanct, even when it was used as part of a cover-up?  How does this work?  We'd appreciate a brief that "briefly" explains it, and I'm sure Phil would, too.