I have modified some details in the text of the file and now I am getting this error. I already installed the corresponding package and nothing ... :(

The error occurs on the line below:

\item $p \to q \,\vdash\, (p \to q) \, \vee \sim r $

Full Code:


  • 1
    Welcome to TeX.SX! Did you see tex.stackexchange.com/q/499082/134574? – Phelype Oleinik Aug 2 '19 at 19:40
  • It should be enough to install the l3backend package through your TeX distribution's package manager (TeX Live's tlmgr or MikTeX Console, make sure to update all installed packages as well, if you are using a mutli-user MikTeX installation update in both Admin and User mode). For future questions please don't link to external download sites for the full code example, instead provide a small example document directly in the question (tex.meta.stackexchange.com/q/228/35864). – moewe Aug 3 '19 at 4:44

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