## TODO

* [x] predicates
* [x] negation
* [x] disjunction
* [x] stratum
* [x] magic sets
* [x] unification
* [x] aggregation
* [x] duplicate symbols in rule heads
* [x] limit, offset
* [x] public API
* [ ] sorting
* [ ] range scan

comparators can have problems when sorting mixed integers and floats