code: func (bh *Header) RemoveReadGroup(rg *ReadGroup) error {
  	if rg.id < 0 || int(rg.id) >= len(bh.refs) || bh.rgs[rg.id] != rg {
  		return errInvalidReadGroup
  	}
  	bh.rgs = append(bh.rgs[:rg.id], bh.rgs[rg.id+1:]...)
  	for i := range bh.rgs[rg.id:] {
  		bh.rgs[i+int(rg.id)].id--
  	}
  	rg.id = -1
  	delete(bh.seenGroups, rg.name)
  	return nil
  }
docs: // RemoveReadGroup removes rg from the Header and makes it
  // available to add to another Header.