code: func (p *MemPostings) Iter(f func(labels.Label, Postings) error) error {
  	p.mtx.RLock()
  	defer p.mtx.RUnlock()
  
  	for n, e := range p.m {
  		for v, p := range e {
  			if err := f(labels.Label{Name: n, Value: v}, newListPostings(p...)); err != nil {
  				return err
  			}
  		}
  	}
  	return nil
  }
docs: // Iter calls f for each postings list. It aborts if f returns an error and returns it.