Slice the mapping by the position in the sequence

       	First coordinate is 0-indexed start
       	Second coordinate is 1-indexed finish

def slice_sequence(self,start,end,directionless=False):
     """Slice the mapping by the position in the sequence

       	First coordinate is 0-indexed start
       	Second coordinate is 1-indexed finish

     """
     if end > self.length: end = self.length
     if start < 0: start = 0
     if not directionless and self.direction == '-': 
        newend = self.length-start
        newstart = self.length-end
        end = newend
        start = newstart
     #find the sequence length
     l = self.length
     indexstart = start
     indexend = end
     ns = []
     tot = 0
     for r in self._rngs:
        tot += r.length
        n = r.copy()
        if indexstart > r.length:
           indexstart-=r.length
           continue
        n.start = n.start+indexstart
        if tot > end:
           diff = tot-end
           n.end -= diff
           tot = end
        indexstart = 0
        ns.append(n)
        if tot == end: break
     if len(ns)==0: return None
     return Transcript(ns,self._options)