Implementation alternative
--------------------------

A more complicated alternative would be to split the ``task_list%update``
into several task sections; e.g., one for guard zone downloading, another
one for the task update, etc., while embracing them all with a
``!omp target data`` / ``!$omp end target data`` pair, allowing data
to survice on the GPU between target sections.

Currently, there are no obvious arguments to choose the more complicated
solution.

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