code: func Merge(its ...Postings) Postings {
  	if len(its) == 0 {
  		return EmptyPostings()
  	}
  	if len(its) == 1 {
  		return its[0]
  	}
  
  	p, ok := newMergedPostings(its)
  	if !ok {
  		return EmptyPostings()
  	}
  	return p
  }
docs: // Merge returns a new iterator over the union of the input iterators.