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. The goal of finding a derivative for the expression X with respect to X itself is satisfied by the result 1. In any program, the procedure for a particular predicate is the sequence of clauses in the program whose head goals have that predicate as principal functor. In Prolog, several predicates may have the same name but different arities.