code: func MustGetBlobRef(req *http.Request, param string) blob.Ref {
  	br, ok := blob.Parse(MustGet(req, param))
  	if !ok {
  		panic(InvalidParameterError(param))
  	}
  	return br
  }
docs: // MustGetBlobRef returns a non-nil BlobRef from req, as given by param.
  // If it doesn't, it panics with a value understood by Recover or RecoverJSON.