code: func NewUnorderedMemPostings() *MemPostings {
  	return &MemPostings{
  		m:       make(map[string]map[string][]uint64, 512),
  		ordered: false,
  	}
  }
docs: // NewUnorderedMemPostings returns a memPostings that is not safe to be read from
  // until ensureOrder was called once.