code: func (b *DescribedBlob) PeerBlob(br blob.Ref) *DescribedBlob {
  	if b.Request == nil {
  		return &DescribedBlob{BlobRef: br, Stub: true}
  	}
  	b.Request.mu.Lock()
  	defer b.Request.mu.Unlock()
  	return b.peerBlob(br)
  }
docs: // PeerBlob returns a DescribedBlob for the provided blobref.
  //
  // Unlike DescribedBlobStr, the returned DescribedBlob is never nil.
  //
  // If the blob was never loaded along with the the receiver (or if the
  // receiver is nil), a stub DescribedBlob is returned with its Stub
  // field set true.