The agenda is essentially a stack of rule groups (called contexts) which are still awaiting execution. The initial invocation of the forward chainer and any subsequent rule can cause contexts to be added to the top of the agenda. During normal execution the forward chainer simply proceeds down the agenda context by context. When the agenda is empty, passing control on will terminate the execution of the rule interpreter. This is a proper way to exit the forward chainer.
KnowledgeWorks and Prolog User Guide (Unix version) - 6 Dec 2011