code: func (stub *MockStub) MockTransactionStart(txid string) {
  	stub.TxID = txid
  	stub.setSignedProposal(&pb.SignedProposal{})
  	stub.setTxTimestamp(util.CreateUtcTimestamp())
  }
docs: // Used to indicate to a chaincode that it is part of a transaction.
  // This is important when chaincodes invoke each other.
  // MockStub doesn't support concurrent transactions at present.