Rules

A rule takes the form of

		 $H$  :- $B_1,B_2,\ldots,B_n.\ (n>0)$

where $H$, $B_1$, $\ldots$, $B_n$ 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 form 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.