std::function<void (event &)>
   kernel_op(cl_command_queue q, cl_kernel kern,
             const std::vector<size_t> &grid_offset,
             const std::vector<size_t> &grid_size,
             const std::vector<size_t> &block_size) {
      const std::vector<size_t> reduced_grid_size = map(
         std::divides<size_t>(), grid_size.begin(), grid_size.end(),
         block_size.begin());

      return [=](event &) {
         kern->launch(*q, grid_offset, reduced_grid_size, block_size);
      };
   }


-----------------------------

hard_event *hev = new hard_event(
      *q, CL_COMMAND_NDRANGE_KERNEL, { deps, deps + num_deps },
      kernel_op(q, kern, grid_offset, grid_size, block_size));


------------------------------------

hard_event::hard_event(clover::command_queue &q, cl_command_type command,
                       std::vector<clover::event *> deps, action action) :
   _cl_event(q.ctx, deps, action, [](event &ev){}),
   __queue(q), __command(command), __fence(NULL) {
   q.sequence(this);
   trigger();
}


-------------------------------------------

_cl_event::_cl_event(clover::context &ctx,
                     std::vector<clover::event *> deps,
                     action action_ok, action action_fail) :
   ctx(ctx), __status(0), wait_count(1),
   action_ok(action_ok), action_fail(action_fail) {
   for (auto ev : deps)
      ev->chain(this);
}

-------------------------------------------------

void
_cl_event::trigger() {
   if (!--wait_count) {
      action_ok(*this);

      while (!__chain.empty()) {
         __chain.back()->trigger();
         __chain.pop_back();
      }
   }
}