Returns a counter object; contains for each key of the mapping the counts
    of the respective value element (= set length).

    :param mapping: dict, for each key contains a set of entries.
    :returns: a counter

def _getValueCounts(mapping):
    """Returns a counter object; contains for each key of the mapping the counts
    of the respective value element (= set length).

    :param mapping: dict, for each key contains a set of entries.
    :returns: a counter
    """
    return Counter({k: len(v) for k, v in viewitems(mapping)})