It corresponds roughly to a verb in natural language, or to a procedure name in a conventional programming language. A logic program consists simply of a sequence of statements called sentences , which are analogous to sentences in natural language. A sentence comprises a head and a body . The head either consists of a single goal or is empty. If the head is not empty, the sentence is called a clause .
P is true.
Goal P is satisfied. P is true if Q and R and S are true. Sentences generally contain variables. A variable should be thought of as standing for some definite but unidentified object. This is analogous to the use of a pronoun in natural language.