'Example:
>>> c = Counter({"a": 4, "b": 2})'

def __init__(units, dimensions, positive):

    for (dimension, count) in dimensions.iteritems():
        if (not isinstance(count, int)):
            raise DimensionError(('%s    dimension    count    must    be    a    whole    number' % dimension))
    units._dimensions = dimensions.copy()
    units._positive = bool(positive)
