This is XeTeX, Version 3.14159265-2.6-0.99996 (TeX Live 2016/Debian) (preloaded format=xelatex 2018.8.12)  26 MAR 2019 13:38
entering extended mode
 restricted \write18 enabled.
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**modern_R.tex
(./modern_R.tex
LaTeX2e <2017/01/01> patch level 3
Babel <3.9r> and hyphenation patterns for 83 language(s) loaded.

! LaTeX Error: File `gitbook.cls' not found.

Type X to quit or <RETURN> to proceed,
or enter new name. (Default extension: cls)

Enter file name: 
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<read *> 
         
l.2 \usepackage
               {lmodern}^^M 
Here is how much of TeX's memory you used:
 9 strings out of 493068
 103 string characters out of 6137720
 55823 words of memory out of 5000000
 4027 multiletter control sequences out of 15000+600000
 3640 words of font info for 14 fonts, out of 8000000 for 9000
 1347 hyphenation exceptions out of 8191
 10i,0n,7p,52b,8s stack positions out of 5000i,500n,10000p,200000b,80000s

No pages of output.