Programming in XPCE/Prolog: Section 2.3

## 2.3 Optional arguments

Arguments to XPCE methods are identified by their position. Many methods have both obligatory and optional arguments. If obligatory arguments are omitted XPCE will generate an error. If optional arguments are omitted XPCE fills the argument with the constant @default. The interpretation of @default is left to the implementation of the receiving method. See also chapter 3.