code: func (b *DescribedBlob) peerBlob(br blob.Ref) *DescribedBlob {
  	if peer, ok := b.Request.m[br.String()]; ok {
  		return peer
  	}
  	return &DescribedBlob{Request: b.Request, BlobRef: br, Stub: true}
  }
docs: // version of PeerBlob when b.Request.mu is already held.