code: func (p *MemPostings) Get(name, value string) Postings {
  	var lp []uint64
  	p.mtx.RLock()
  	l := p.m[name]
  	if l != nil {
  		lp = l[value]
  	}
  	p.mtx.RUnlock()
  
  	if lp == nil {
  		return EmptyPostings()
  	}
  	return newListPostings(lp...)
  }
docs: // Get returns a postings list for the given label pair.