diff --git a/TODO b/TODO
index aa071a8d..28052034 100644
--- a/TODO
+++ b/TODO
@@ -18,6 +18,118 @@
 - YYERRCODE, YYUNDEF, YYEOF
 - YYERRCODE and translation
 
+** yypcontext_expected_tokens
+Beware that returning 0 is unclear: does it mean there are no possible
+lookahead, or that there are too many?
+
+** YYERRCODE
+How about using "return YYERRCODE" to tell the parser to enter
+error-recovery, but not emit an error message?  I have frequently felt the
+need for the scanner to be able to do that.  In C++ we can, thanks to throw
+syntax_error.
+
+Some people seem to be aware that return YYERRCODE is possible...
+https://github.com/search?q=%22return+YYERRCODE%22&type=Code
+
+*** unixODBC
+There are tons of copies on GitHub, I don't know where the original is.
+
+https://github.com/faiz-akhtar/Calico/blob/01b49089a3eb535380877bbdc6c641b8b4e7ca6f/Drivers/nn/yylex.c
+
+But they define YYERRCODE themselves!  They are stepping on "our" namespace.
+
+```
+# define	YYERRCODE	256
+```
+
+Besides, what they do is super-weird: they use YYERRCODE is several places
+for their own error tracking.
+
+```
+int	nnsql_yylex(YYSTYPE* pyylval, yyenv_t* penv)
+{
+[...]
+                len = getname(penv->texts_bufptr, -1, penv);
+
+                if( len == YYERRCODE )
+                        return len;
+```
+
+*** avrdude-6.3
+
+Latest version: http://svn.savannah.gnu.org/viewvc/avrdude/trunk/avrdude/lexer.l?view=markup
+
+We can certainly talk to them about their use of YYERRCODE.
+
+Replicas on GitHub:
+
+https://github.com/tam1035/Phys605-March27/blob/0f856baf20877b6f333f91ed17860bd752c7d68e/AVR/avrdude-6.3/lexer.l
+
+They emit error messages from the scanner and force error recovery.  But
+then, they get two error messages.
+
+```
+c: { yyerror("possible old-style config file entry\n"
+             "  Update your config file (see " CONFIG_DIR
+               "/avrdude.conf.sample for a sample)");
+     return YYERRCODE; }
+```
+
+Unfortunately they also expect the error message at some places.  Here,
+return YYUNDEFTOK (or just return yytext[0]) would have been better.
+
+```
+. { return YYERRCODE; }
+```
+
+*** Casacore  A suite of C++ libraries for radio astronomy data processing.
+
+https://github.com/casacore/casacore/blob/110bf435c3fe292ee0c3e4bdd039853d257c9536/ms/MSSel/MSAntennaGram.ll
+
+```
+ /* Any other character is invalid */
+.         { return YYERRCODE; }
+```
+
+A pattern they used several times.
+https://github.com/casacore/casacore/search?q=YYERRCODE&unscoped_q=YYERRCODE
+
+I don't understand how this works: AFAICT, we only define YYERRCODE in
+y.tab.c, not y.tab.h.  That's the case in Apple's Bison 2.3 for instance.
+Flex does not generate it either.
+
+*** Some shell
+Seems to be old versions of busybox.
+
+https://github.com/luotao717/arsdk/blob/9f9004157afcd26b915125576a815f6b3e0cd715/apps/busybox-1.01/shell/msh.c
+
+
+```
+#define	YYERRCODE 300
+```
+
+Used only in one function (collect)
+
+But today the parser is written by hand.
+
+https://github.com/mirror/busybox/blob/master/shell/ash.c#L11527
+
+*** glibc's intl
+
+https://sourceware.org/git/?p=glibc.git;a=blob;f=intl/plural.y;h=f69fba7c602e9f890f8e5c3b2cd7fe1c487714ea;hb=HEAD
+
+Their yylex returns YYERRCODE, but they don't want any error message.
+
+```
+ 379 static void
+ 380 yyerror (struct parse_args *arg, const char *str)
+ 381 {
+ 382   /* Do nothing.  We don't print error messages here.  */
+ 383 }
+```
+
+This code was not updated in years.
+
 ** Java
 *** Examples
 Have an example with a push parser.  Use autocompletion in that case.