void unsubscribe() {
      pendingTransactions.unsubscribePendingTransactions(onAddedListenerId);
      pendingTransactions.unsubscribeDroppedTransactions(onDroppedListenerId);
    }
Maintains the set of pending transactions received from JSON-RPC or other nodes. Transactions are
removed automatically when they are included in a block on the canonical chain and re-added if a
re-org removes them from the canonical chain again.

<p>This class is safe for use across multiple threads.
ethereum/eth/src/main/java/org/hyperledger/besu/ethereum/eth/transactions/TransactionPool.java