'Returns a sequence of [(t1,t2), txt] covering all the given subclip
from t_start to t_end. The first and last times will be cropped so as
to be exactly t_start and t_end if possible.'

def in_subclip(self, t_start=None, t_end=None):

    def is_in_subclip(t1, t2):
        try:
            return ((t_start <= t1 < t_end) or (t_start < t2 <= t_end))
        except:
            return False
    def try_cropping(t1, t2):
        try:
            return (max(t1, t_start), min(t2, t_end))
        except:
            return (t1, t2)
    return [(try_cropping(t1, t2), txt) for ((t1, t2), txt) in self.subtitles if is_in_subclip(t1, t2)]
