code: func (dr *DescribeRequest) noteResultFromRule(rule *DescribeRule, br blob.Ref) {
  	if dr.resFromRule == nil {
  		dr.resFromRule = make(map[*DescribeRule]map[blob.Ref]bool)
  	}
  	m, ok := dr.resFromRule[rule]
  	if !ok {
  		m = make(map[blob.Ref]bool)
  		dr.resFromRule[rule] = m
  	}
  	m[br] = true
  }
docs: // dr.mu just be locked.