This is pdfTeX, Version 3.141592653-2.6-1.40.23 (TeX Live 2022/dev) (preloaded format=pdflatex 2022.3.1) 20 OCT 2022 22:04 entering extended mode \write18 enabled. file:line:error style messages enabled. %&-line parsing enabled. **apx-checking.tex (./apx-checking.tex LaTeX2e <2021-11-15> patch level 1 L3 programming layer <2022-02-24> ./apx-checking.tex:1: Undefined control sequence. l.1 \subsection {Communicating finite state machines\label{app:cfsm}} ? ./apx-checking.tex:1: Emergency stop. l.1 \subsection {Communicating finite state machines\label{app:cfsm}} End of file on the terminal! Here is how much of TeX's memory you used: 16 strings out of 478276 411 string characters out of 5846491 289381 words of memory out of 5000000 18324 multiletter control sequences out of 15000+600000 469259 words of font info for 28 fonts, out of 8000000 for 9000 1141 hyphenation exceptions out of 8191 13i,0n,12p,112b,10s stack positions out of 5000i,500n,10000p,200000b,80000s ./apx-checking.tex:1: ==> Fatal error occurred, no output PDF file produced!