macros - Defined command only works in preamble - TeX - LaTeX Stack Exchange
Rohan Alexander: Getting started with LaTeX
koma script - Move chapter preamble before Chapter headline - TeX - LaTeX Stack Exchange
LaTeX Error: Can be used only in preamble. · Issue #1 · HuangCongQing/LaTeX · GitHub
Hadley Wickham on Twitter: "@keeghin any latex in Rmarkdown is passed through unmodified, so you can just use the regular latex command" / Twitter
Adding Commands to the Preamble
GitHub - SSoelvsten/latex-preamble-and-examples: A LaTeX preample for Computer Science handins and presentations (Danish and English) with reverse-engineerable examples