docs: def slice_target(self,chr,start,end):
       """Slice the mapping by the target coordinate
          
          First coordinate is 0-indexed start
          Second coordinate is 1-indexed finish
  
       """
       # create a range that we are going to intersect with
       trng = Bed(chr,start,end)
       nrngs = []
       for r in self._rngs:
          i = r.intersect(trng)
          if not i: continue
          nrngs.append(i)
       if len(nrngs) == 0: return None
       return MappingGeneric(nrngs,self._options)