Is unwanted backtracking hurting your application performance? Getting some puzzling answers that should not be there? SICStus Prolog includes a determinacy checker, spdet, originally written by Dave Bowen and Peter Schachte, for spotting non-deterministic predicates. A must have tool. Have you tried it recently in your code? You might be surprised by the results.

ECLiPSe 6.x, recently released, features an improved compiler, which, among other goodies, improves detection of singleton variables:

Additional singleton warnings: The new compiler will detect additional cases of singleton variables, namely those that occur only once in a branch of a disjunction.

This may sound like a mandatory feature until you realize that most other Prolog compilers fail to detect such cases of singleton variables.