Constructs an event rule from the factory api.

def make_eventrule(date_rule, time_rule, cal, half_days=True):
    """
    Constructs an event rule from the factory api.
    """

    # Insert the calendar in to the individual rules
    date_rule.cal = cal
    time_rule.cal = cal

    if half_days:
        inner_rule = date_rule & time_rule
    else:
        nhd_rule = NotHalfDay()
        nhd_rule.cal = cal
        inner_rule = date_rule & time_rule & nhd_rule

    return OncePerDay(rule=inner_rule)