-The trace facility uses @code{stderr}, so you must add
-@w{@code{#include <stdio.h>}} to the prologue unless it is already there.
+The trace facility outputs messages with macro calls of the form
+@code{YYFPRINTF (YYSTDERR, @var{format}, @var{args})} where
+@var{format} and @var{args} are the usual @code{printf} format and
+arguments. If you define @code{YYDEBUG} but do not define
+@code{YYFPRINTF}, @code{<stdio.h>} is automatically included and the
+macros are defined to @code{fprintf} and @code{stderr}. In the same
+situation, C++ parsers include @code{<cstdio.h>} instead, and use
+@code{std::fprintf} and @code{std::stderr}.