XSB extends the standard functionality of Prolog (being a

descendant of PSB- and SB-Prolog) to include implementations

of OLDT (tabling) and HiLog terms. OLDT resolution is

extremely useful for recursive query computation, allowing

programs to terminate correctly in many cases where Prolog

does not. HiLog supports a type of higher-order programming

in which predicate symbols can be variable or structured. This

allows unification to be performed on the predicate symbols

themselves in addition to the arguments of the predicates. Of

course, Tabling and HiLog can be used together.