presto-main/src/main/java/io/prestosql/execution/StartTransactionTask.java/StartTransactionTask/convertLevel
class StartTransactionTask:    private static IsolationLevel convertLevel(Isolation.Level level)
    {
        switch (level) {
            case SERIALIZABLE:
                return IsolationLevel.SERIALIZABLE;
            case REPEATABLE_READ:
                return IsolationLevel.REPEATABLE_READ;
            case READ_COMMITTED:
                return IsolationLevel.READ_COMMITTED;
            case READ_UNCOMMITTED:
                return IsolationLevel.READ_UNCOMMITTED;
            default:
                throw new AssertionError("Unhandled isolation level: " + level);
        }
    }