+xbt::signal<void()> SafetyChecker::on_exploration_start_signal;
+xbt::signal<void()> SafetyChecker::on_backtracking_signal;
+
+xbt::signal<void(State*)> SafetyChecker::on_state_creation_signal;
+
+xbt::signal<void(State*)> SafetyChecker::on_restore_system_state_signal;
+xbt::signal<void()> SafetyChecker::on_restore_initial_state_signal;
+xbt::signal<void(Transition*)> SafetyChecker::on_transition_replay_signal;
+xbt::signal<void(Transition*)> SafetyChecker::on_transition_execute_signal;
+
+xbt::signal<void()> SafetyChecker::on_log_state_signal;
+