Oleg Kiselyov's minimal implementation of logic programming.
From Oleg's description at http://okmij.org/ftp/Scheme/misc.html#sokuza-kanren :
This code develops a minimal logic programming system: just enough to get the taste of the concept. The over-arching idea is logic programming being the combination of logic variables and 'non-deterministic functions' (that is, 'functions' that can return more or fewer than one result). We also make an attempt at the multiple-world semantics of logic programs, and the treatment of logic variables as quantum mechanical quantities, with unification being 'the measurement'. This code is an attempt to distill Kanren to the simplest terms.