'Adds the statistics of two drives together.
Note that once two drives are added, the following fields
automatically get None values: result, field_start, field_end,
time_start and time_end.'

def __add__(self, other):

    assert (self.team == other.team), ('Cannot    add    drives    from    different    teams    "%s"    and    "%s".' % (self.team, other.team))
    new_drive = Drive(None, 0, '', None)
    new_drive.team = self.team
    new_drive.home = self.home
    new_drive.first_downs = (self.first_downs + other.first_downs)
    new_drive.penalty_yds = (self.penalty_yds + other.penalty_yds)
    new_drive.total_yds = (self.total_yds + other.total_yds)
    new_drive.pos_time = (self.pos_time + other.pos_time)
    new_drive.play_cnt = (self.play_cnt + other.play_cnt)
    new_drive.__plays = (self.__plays + other.__plays)
    new_drive.result = None
    new_drive.field_start = None
    new_drive.field_end = None
    new_drive.time_start = None
    new_drive.time_end = None
    return new_drive
