@classmethod
    def fold(cls, lst, now):
        return [(o.key,o.value,o.flag if o.flag else now) for o in lst]
implements the interval functionality for expressions
dataset/ETHPy150Open kiip/statsite/statsite/metrics.py/Metric.fold--crossover with--dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.5/django/db/backends/postgresql_psycopg2/operations.py/DatabaseOperations.date_interval_sql