def _get_n_highest(n):
    """ Return a function which gets two n-tuples and return one n-tuple which
    contains the highest values. Ignore Nones. """
    def n_highest(a, b):
        merged = [x for x in a + b if x is not None]
        highest = sorted(merged)[-n:]
        if len(highest) < n:
            highest += [None] * (n - len(highest))
        return tuple(highest)
    return n_highest