presto-main/src/main/java/io/prestosql/execution/StartTransactionTask.java/StartTransactionTask/extractIsolationLevel
class StartTransactionTask:    private static Optional<IsolationLevel> extractIsolationLevel(StartTransaction startTransaction)
    {
        if (startTransaction.getTransactionModes().stream()
                .filter(Isolation.class::isInstance)
                .count() > 1) {
            throw semanticException(SYNTAX_ERROR, startTransaction, "Multiple transaction isolation levels specified");
        }

        return startTransaction.getTransactionModes().stream()
                .filter(Isolation.class::isInstance)
                .map(Isolation.class::cast)
                .map(Isolation::getLevel)
                .map(StartTransactionTask::convertLevel)
                .findFirst();
    }