SWI-Prolog version 1.4 was the first release to support garbage collection. Together with tail-recursion optimisation this guaranties forward chaining programs do not waste indefinite amounts of memory. Previous releases of this manual stressed on using failure-driven loops in those cases that no information needed to be passed to the next iteration via arguments. This to avoid large amounts of garbage. This is no longer strictly necessary, but it should be noticed that garbage collection is a time consuming activity. Failure driven loops tend to be faster for this reason. The garbage collector is deactivated when Prolog is called back from a foreign language predicate. This implies there is no garbage collection within a break environment. More seriously, there is no garbage collection when handling call-backs from ---for example--- the XPCE package.