Builtin predicates generates exceptions using a term errorFormal, Context. The first argument is the `formal' description of the error, specifying the class and generic defined context information. When applicable, the ISO error-term definition is used. The second part describes some additional context to help the programmer while debugging. In its most generic form this is a term of the form contextName/Arity, Message, where Name/Arity describes the built-in predicate that raised the error, and Message provides an additional description of the error. Any part of this structure may be a variable if no information was present.