'at(timestamp) -> SubRipFile clone
timestamp argument should be coercible to SubRipFile object.
A specialization of slice. Return all subtiles visible at the
timestamp mark.
Example:
>>> subs.at((0, 0, 20, 0)).shift(seconds=2)
>>> subs.at(seconds=20).shift(seconds=2)'

def at(self, timestamp=None, **kwargs):

    time = (timestamp or kwargs)
    return self.slice(starts_before=time, ends_after=time)
