code: func (dr *DescribeRequest) DescribedBlobStr(blobstr string) *DescribedBlob {
  	if dr == nil {
  		return nil
  	}
  	dr.mu.Lock()
  	defer dr.mu.Unlock()
  	return dr.m[blobstr]
  }
docs: // DescribedBlobStr when given a blobref string returns a Description
  // or nil.  dr may be nil itself.