Clauses

Prolog programs are written as a sequence of clauses. Each has the form

predicate( argument, argument, ... ).

predicate( argument, argument, ... ) :-
    predicate( argument, argument, ... ),
    predicate( argument, argument, ... ),
    ... .

, means ``and''.

:- means ``if''.

All clauses are Horn clauses: one goal, implied by a conjunction of other goals (possibly none).