Add a placeholder for [diff.cpp20].

Author: zygoloid

source/compatibility.tex

@@ -1,6 +1,16 @@
 %!TEX root = std.tex
 \infannex{diff}{Compatibility}
 
+% TODO: Add this once we have differences.
+
+%\rSec1[diff.cpp20]{\Cpp{} and ISO \CppXX{}}
+%
+%\pnum
+%\indextext{summary!compatibility with ISO \CppXX{}}%
+%This subclause lists the differences between \Cpp{} and
+%ISO \CppXX{} (ISO/IEC 14882:2020, \doccite{Programming Languages --- \Cpp{}}),
+%by the chapters of this document.
+
 \rSec1[diff.cpp17]{\Cpp{} and ISO \CppXVII{}}
 
 \pnum

tools/check.sh

@@ -23,7 +23,7 @@ for f in *.tex; do [ $(tail -c 2 $f | wc -l) -eq 1 ] || (echo "$f has trailing e
 grep -ne '^.\+\\\(begin\|end\){codeblock}' $texfiles && exit 1
 
 # \pnum not alone on a line.
-grep -ne '^.\+\\pnum' $texfiles && exit 1
+grep -ne '^[^%]\+\\pnum' $texfiles && exit 1
 grep -ne '\\pnum.\+$' $texfiles && exit 1
 # Fixup: sed '/\\pnum.\+$/s/\\pnum\s*/\\pnum\n/'