Spy points are the most important debugging facility in Common Prolog. They are used in the same way trace is used in Lisp. After executing (spy foo), all events associated with satisfying foo goals will be traced and the user will enter a debugging command loop at every port (see Interactive Debugging below). A user can also specify (spy (foo 3)), (spy (foo bar)), or (spy ((foo 3) bar)) to place spy points on foo goals with arity 3, on all predicates for foo and bar, or on foo with arity 3 and all predicates for bar respectively. Spy points are turned off with (nospy <spypoints>). If no spy points are mentioned, nospy will turn off all spy points.
KnowledgeWorks and Prolog User Guide (Unix version) - 26 Feb 2015