KANREN is a declarative logic programming system with first-class relations, embedded in a pure functional subset of Scheme. The system has a set-theoretical semantics, true unions, fair scheduling, first-class relations, lexically-scoped logical variables, depth-first and iterative deepening strategies. The system achieves high performance and expressivity without cuts.
kanren sourceforge site
I haven't looked very closely at this yet, but the fact that Oleg and Daniel Friedman are involved strongly suggests that I should.