title: U-SQL Error - Change the identifier to use at least one lower case letter
body: <p>I am fairly new to U-SQL and trying to run a U-SQL script in Azure Data Lake Analytics to process a parquet file using the Parquet extractor functionality. I am getting the below error and I don't find a way to get around it. 
  Error - Change the identifier to use at least one lower case letter. If that is not possible, then escape that identifier (for example: '[ACTIVITY]'), or embed it in a CSHARP() block (e.g CSHARP(ACTIVITY)).</p>
  
  <p>Unfortunately all the different fields generated in the Parquet file are capitalized and I don't want to to escape these identifiers. I have tried if I could wrap the identifier with CSHARP block and it fails as well (E_CSC_USER_RESERVEDKEYWORDASIDENTIFIER: Reserved keyword CSHARP is used as an identifier.)  Is there anyway I could extract the parquet file? Thanks for your help!
  Code Snippet:</p>
  
  <pre><code>SET @@FeaturePreviews = "EnableParquetUdos:on";
  </code></pre>
  
  <p>@var1 =
      EXTRACT ACTIVITY string,
              AUTHOR_NAME string,
              AFFLIATION string</p>
  
  <pre><code>FROM "adl://xxx.azuredatalakestore.net/Abstracts/FY2018_028"
  USING Extractors.Parquet();
  </code></pre>
  
  <p>@var2 =
       SELECT *
      FROM @var1 
      ORDER BY ACTIVITY ASC
      FETCH 5 ROWS;<br>
  OUTPUT @var2<br>
  TO "adl://xxx.azuredatalakestore.net/Results/AbstractsResults.csv"
  USING Outputters.Csv();</p>