## 4.1 Notation of Predicate Descriptions

We have tried to keep the predicate descriptions clear and concise. First the predicate name is printed in bold face, followed by the arguments in italics. Arguments are preceded by a `+', `-' or `?' sign. `+' indicates the argument is input to the predicate, `-' denotes output and `?' denotes `either input or output'.20These marks do not suggest instantiation (e.g. var(+Var)). Constructs like `op/3' refer to the predicate `op' with arity `3'. Finally, arguments may have the `:' specifier, which implies the argument is module-sensitive. Normally the argument is a callable term referring to a predicate in a specific module. See section 5 for more information on module-handing.