Clauses

The syntax of $\alpha$Prolog program clauses is summarized by the following grammar:

$$P ::= A \mid A \mathrel{{:}{-}}G \mid A \longrightarrow D$$

The first two forms are Horn clauses (expressed in implicational form). That is, they are either a single predicate (a fact) or a predicate qualified by a condition $G$ (a rule). The third form is a definite clause grammar rule, which says that the nonterminal synbol $A$ can be rewritten to the DCG body $D$.