code: func (x *Index) isDeleted(br blob.Ref) bool {
  	deletes, ok := x.deletes.m[br]
  	if !ok {
  		return false
  	}
  	for _, v := range deletes {
  		if !x.isDeleted(v.deleter) {
  			return true
  		}
  	}
  	return false
  }
docs: // The caller must hold x.deletes.mu for read.