Objects denoting numbers or strings are different from other objects because the usual comparison operators are defined for them, as well as several arithmetic functions. Within a query or a rule body, relations between numbers or strings may be tested with the comparison expressions. For example, the following query asks for all car owners younger than 22:
?- ?X:Car[owner->?P] AND ?P[age->?A] AND ?A < 22.
The arithmetic operations addition +, subtraction -, multiplication * and division / are also implemented. Arithmetic expressions may be constructed in the usual way. Even complex expressions, e.g. 3 + 5 + 2 or 3 + 2 * 3 are supported. By default, multiplication and division have a higher precedence than addition and subtraction. As usual, the evaluation order may be changed using parentheses, e.g. (3 + 2) * 3. The following example contains the query that computes the average age of Peter and Paul.
(?A is (?P1+?P2)/2.0) .
Additionally the following mathematical functions are implemented: