def _sum_counters(*counters_list):
    """Combine many maps from group to counter to amount."""
    result = {}

    for counters in counters_list:
        for group, counter_to_amount in counters.items():
            for counter, amount in counter_to_amount.items():
                result.setdefault(group, {})
                result[group].setdefault(counter, 0)
                result[group][counter] += amount

    return result