Gerona Forward Chaining Gerona Forward Chaining is a forward-chaining rule execution technique that does not backtrack and that executes rules in the same scope in parallel. Gerona rules are associated with each event the real-time, crisis decision-making system receives. Rules are executed in parallel without backtracking. If a rule's precondition succeeds, its postcondition is executed; the postcondition may contain additional nested rules, which are also executed in parallel.