'Iterates over list checking segments with same sort do not overlap
:raise: Exception: if segments overlap space with same sort'

def _debug_check(self):

    old_end = 0
    old_sort = ''
    for segment in self._list:
        if ((segment.start <= old_end) and (segment.sort == old_sort)):
            raise Exception('Error    in    SegmentList:    blocks    are    not    merged')
        old_end = segment.end
        old_sort = segment.sort
