The ETL as a default uses #include headers guards.
If you wish, you can convert them to using #pragma once.

Install guardonce

python3 -m pip install guardonce

Modify all of the headers

Navigate to etl/scripts

To change from #include guard to #pragma once ./convert_headers_to_use_once.bat

To change from #pragma once to #include guard ./convert_headers_to_use_guards.bat