C++ alternative tokens in the preprocessor

[Martin v. Loewis]

> This is not a good error message.  It expects the user to know gritty
> details of the official grammar.  I would much rather see something
> like
> 
> a.cc:1:9: "and" is not a legal macro name in C++

This is slightly off-topic now, but I was going to make the comment
that I feel such an error message is inappropriate; in particular,
that use of the adjective "legal" is.

I believe it was RMS who once mentioned that the compiler should not
declare things "illegal"
( http://gcc.gnu.org/ml/gcc/1999-09n/msg00395.html ), with "not legal"
probably being just as bad.

I always used "ill-formed", to describe the code and not its authors,
but that may be a bit technical.

I'd appreciate comments and recommendations from native speakers of
the English language what terminology would be acceptable.

Regards,
Martin