Looks like this is a direct port of pg's version. I have to admit, I haven't seen callcc before and the concept while similar to closures scares me a bit. Storing a complete environment in a variable is a new concept to wrap my head around.<p><pre><code> This is an all-ruby implementation of choose/fail nondeterministic
programming with branch cut, as described in Chapter 22 of Paul Graham’s
On Lisp[1], or Section 4.3 of SICP[2].
Due to Ruby containing a true call/cc, this is a much straighter port
of Paul Graham’s scheme version of this code than his Common Lisp or
my C versions are. :-)</code></pre>