panic(fmt.Sprintf("invalid size %d (should be >= 0)", size))
	}
	// base >= s.base && size >= 0
	base += size + 1 // +1 because EOF also has a position
	if base < 0 {
		panic("token.Pos offset overflow (> 2G of source code in file set)")
	}
	// add the file to the file set
	s.base = base
	s.files = append(s.files, f)
	s.last.Store(f)
	return f
}