. . .
I was hoping that the examples I requested would be examples of particular control constructs or extensions to the language's syntax and semantics. Though I admit that such things are possible in lisp, I suspect that their utility is minimal.
As to utility, quite the contrary, I think. Offhand I can think of the screamer package for Common Lisp, which provides non-deterministic mechanisms for use in backtracking applications. For a while in the 80's there was practically a cottage industry implementing various flavors of Prolog and other Logic Programming languages in Lisp; one notable example was LogLisp. I think many of the more advanced constructs in CL were originally macro extensions to the earlier lisps; e.g. structures, objects and classes, the LOOP macro, streams and iterators, generalized setters and getters. Actors, which was one of the ancestors of OOP, was first as a Lisp extension. In the AI hayday of the mid-80's most of the expert system shells, providing forward and backward chaining mechanisms, frames and semantic nets, and object-centered and data-driven programming, were originally implemented as packages integrated into Lisp. All of these made non-trivial extensions to Lisp, and all were of arguably great utility. -- Bill Wood bill.wood@acm.org