build/transaction_envelope.go
func (m *TransactionBuilder) MutateTransactionEnvelope(txe *TransactionEnvelopeBuilder) error {
	if m.Err != nil {
		return m.Err
	}

	txe.E.Tx = *m.TX
	newChild := *m
	txe.child = &newChild
	m.TX = &txe.E.Tx
	return nil
}
func (m *TransactionBuilder) MutateTransactionEnvelope(txe *TransactionEnvelopeBuilder) error {
	if m.Err != nil {
		return m.Err
	}

	txe.E.Tx = *m.TX
	newChild := *m
	txe.child = &newChild
	m.TX = &txe.E.Tx
	return nil
}
// MutateTransactionEnvelope for TransactionBuilder causes the underylying
// transaction to be set as the provided envelope's Tx field