code: func ExtractEnvelopeOrPanic(block *cb.Block, index int) *cb.Envelope {
  	envelope, err := ExtractEnvelope(block, index)
  	if err != nil {
  		panic(err)
  	}
  	return envelope
  }
docs: // ExtractEnvelopeOrPanic retrieves the requested envelope from a given block
  // and unmarshals it -- it panics if either of these operations fail