[KBK] (25 April 2002) -
The fact that Tcl allows new control structures to be defined at the script level by means of [uplevel] and [upvar] has led to its being a popular platform for prototyping search algorithms that require backtracking.  The general idea is that you can program a proc that makes a nondeterministic choice, and evaluates a script to follow through on the consequences of that choice.

[RS] examines some of the possibilities in [Playing Prolog] and [Playing predicate logic].  [Searching a star in space] is also interesting.

http://www.usenix.org/publications/library/proceedings/tcl96/clark.html