code: func NewMockProducer(ledger Ledger) *MockProducer {
  	c := &MockProducer{
  		rcvch:  make(chan interface{}, 100),
  		ledger: ledger,
  	}
  	go c.listen()
  	ledger.Register(c.rcvch)
  	return c
  }
docs: // NewMockProducer returns a new MockProducer