code: func (tx *Tx) Rehash() []byte {
  	hasher := sha256.New()
  	hasher.Write(tx.MustSignBytes())
  	tx.txHash = hasher.Sum(nil)
  	tx.txHash = tx.txHash[:HashLength]
  	return tx.txHash
  }
docs: // Regenerate the Tx hash if it has been mutated or as called by Hash() in first instance