This work may be found here.
This is a paper describing a technique of efficient stack implementation in a Lisp system, detailing how to cope with the cases where a stack discipline is commonly followed, but cannot be relied upon.
The Phantom Stack method is thought-provoking, bringing to mind other techniques for avoiding costly edge cases, common to older computing; this technique is titular. Stallman's writing style compares it to how the CIA operates, a reflection of an attitude from a time when hackers were truly hackers.
The paper can be read in little time, and so a more detailed review is unwarranted.