code: func (b *Bitmap) OffsetRange(offset, start, end uint64) *Bitmap {
  	if lowbits(offset) != 0 {
  		panic("offset must not contain low bits")
  	}
  	if lowbits(start) != 0 {
  		panic("range start must not contain low bits")
  	}
  	if lowbits(end) != 0 {
  		panic("range end must not contain low bits")
  	}
  
  	off := highbits(offset)
  	hi0, hi1 := highbits(start), highbits(end)
  	citer, _ := b.Containers.Iterator(hi0)
  	other := NewBitmap()
  	for citer.Next() {
  		k, c := citer.Value()
  		if k >= hi1 {
  			break
  		}
  		other.Containers.Put(off+(k-hi0), c)
  	}
  	return other
  }
docs: // OffsetRange returns a new bitmap with a containers offset by start.