if err == nil {
		return b
	}
	b.errs = append(b.errs, err)
	return b
}

// VisitorConcurrency sets the number of concurrent visitors to use when
// visiting lists.
func (b *Builder) VisitorConcurrency(concurrency int) *Builder {
	b.visitorConcurrency = concurrency
	return b
}

// FilenameParam groups input in two categories: URLs and files (files, directories, STDIN)