This is BibTeX, Version 0.99d (TeX Live 2015/Debian)
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The top-level auxiliary file: ug_cat.aux
The style file: plainnat.bst
Database file #1: ref.bib
Warning--I didn't find a database entry for "fraedrichetal1998"
Warning--I didn't find a database entry for "borthetal2016"
You've used 10 entries,
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(There were 2 warnings)