Definite rules

A definite clause in the form $\neg X_1\vee \dots \vee \neg X_m\vee X$ can be rewritten in the form $\underset{\text{body}}{\underbrace{X_1\wedge \dots \wedge X_m}}\to \underset{\text{head}}{\underbrace{X}}$.

In a program, a rule without a body is also called a fact because it states something that we are sure of.