code: func (dr *DescribeRequest) foreachResultBlob(fn func(blob.Ref)) {
  	if dr.BlobRef.Valid() {
  		fn(dr.BlobRef)
  	}
  	for _, br := range dr.BlobRefs {
  		fn(br)
  	}
  	for brStr := range dr.m {
  		if br, ok := blob.Parse(brStr); ok {
  			fn(br)
  		}
  	}
  }
docs: // Requires dr.mu is held.