'Gets the Nth drive where the first drive corresponds to n=1. This is
only useful given a complete collection of drives for an entire game.
If the team parameter is specified (i.e., team=\'NE\'), then n will
be interpreted as *that* team\'s Nth drive.'

def number(self, n, team=None):

    assert (n > 0)
    n -= 1
    if (team is None):
        return list(self)[n]
    else:
        i = 0
        for d in self:
            if (d.team == team):
                if (i == n):
                    return d
                i += 1
        assert False, ('Could    not    find    drive    %d    for    team    %s.' % ((n + 1), team))
