Rules

A rule takes the form of

      H :- B1,B2,...,Bn.  (n>0)

where H, B1, ..., Bn are atomic formulas. H is called the head and the right hand side of :- is called the body of the rule. A fact can be considered a special kind of rule whose body is true.  

A predicate is an ordered sequence of clauses whose heads have the same predicate symbol and the same arity.