/// Enqueue barrier (with wait list) into the queue
inline event enqueue_barrier(command_queue &q,
const wait_list &events = wait_list())
{
context_id ctx = get_context_id(q);
wait_list my_events;
std::copy_if(events.begin(), events.end(),
std::back_inserter(my_events),
[=](const event &e){ return ctx == get_context_id(e); });
event e;
q.enqueueBarrierWithWaitList(&my_events, &e);
return e;
}
