// These will be chosen before others.
func (p *parallelReader) preferReaders(prefer []bool) {
	if len(prefer) != len(p.orgReaders) {
		return
	}
	// Copy so we don't change our input.
	tmp := make([]io.ReaderAt, len(p.orgReaders))
	copy(tmp, p.orgReaders)
	p.readers = tmp
	// next is the next non-preferred index.
	next := 0
	for i, ok := range prefer {
		if !ok || p.readers[i] == nil {
			continue