Constraint Handling Rules. CHR looks pretty interesting; I saw someone describe an implementation of Fibonacci heaps in CHR[1] as "executable pseudocode", and that's generally a sign of something useful.
I agree they're useful, but CHR has been included as a standard part of most Prolog implementations since the 1990s. It's possible it just hasn't been fully discovered yet, but it's had some time to attempt to win.
[1] http://people.cs.kuleuven.be/~jon.sneyers/presentations/dijk...