Are you using Inclusive or Exclusive build expression?
Sounds like it's Inclusive, but thought I'd ask.
If so, you might try using Exclusive. Just clear tags with anything related to A. After all, it's included by default anyway, no? As an aside, if the folder is empty, not really seeing the concern. Just curiosity?
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I think that I'm using "Inclusive" because I list the tags I want to include in the Available tags pane and the tags to exclude in the Exclude from output pane. This is the build expression for "B".
However, the build expression for "B" sounds more 'exclusive' to me.
I'd like to fix this problem because I'm curious and I like to understand what's going on, but also because I like my projects to be as clean as possible. I inherited this project from a writer who is not working here any longer, and who had, I think, imported lots of stuff into this new project in order to carry content forward from a legacy project. My goal is to clean up the project so that future writers won't have to worry about what's going on in the project.