code: def slice_sequence(self,start,end):
       
       #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 MappingGeneric(ns,self._options)
docs: Slice the mapping by the position in the sequence
          
          First coordinate is 0-indexed start
          Second coordinate is 1-indexed finish