code: func (r *Record) Bin() int {
  	if r.Flags&(Unmapped|MateUnmapped) == Unmapped|MateUnmapped {
  		return 4680 // reg2bin(-1, 0)
  	}
  	return int(internal.BinFor(r.Pos, r.End()))
  }
docs: // Bin returns the BAM index bin of the record.