'Convert a field value into a set of series keys
@param value: the field value'

@staticmethod
def series(value):

    if (value is DEFAULT):
        series = set()
    elif (value is None):
        series = set([None])
    elif (type(value) is list):
        series = set(s3_flatlist(value))
    else:
        series = set([value])
    return series
