code: func (mp *TxPool) CheckSpend(op wire.OutPoint) *btcutil.Tx {
  	mp.mtx.RLock()
  	txR := mp.outpoints[op]
  	mp.mtx.RUnlock()
  
  	return txR
  }
docs: // CheckSpend checks whether the passed outpoint is already spent by a
  // transaction in the mempool. If that's the case the spending transaction will
  // be returned, if not nil will be returned.