xbt::signal<void(Actor const&)> s4u::Actor::on_wake_up;
xbt::signal<void(Actor const&)> s4u::Actor::on_migration_start;
xbt::signal<void(Actor const&)> s4u::Actor::on_migration_end;
+xbt::signal<void(Actor const&)> s4u::Actor::on_termination;
xbt::signal<void(Actor const&)> s4u::Actor::on_destruction;
// ***** Actor creation *****