void checked_head_all_failable(Predicate cont_pred) {
if (! lock().owns_lock()) throw LockError();
if (! head(cont_pred)) throw CondError();
}
void checked_head_all_failable_timed(Predicate cont_pred, const Time &time) {
if (! lock().owns_lock()) throw LockError();
if (! timed_head(cont_pred, time)) throw CondError();
}
void checked_head_failable_lock(Predicate cont_pred) {
if (! lock().owns_lock()) throw LockError();
head(cont_pred);
}