ANSI/ISO C99 requires assert() to write to stderr. This means that

ANSI/ISO C99 requires assert() to write to stderr.  This means that
writing to STDERR_FILENO is insufficient, as the user could freopen
stderr.  It is also insufficient to output to fileno(stderr) since
this would fail in the custom stream case.  I didn't remove the
old code though, as it doesn't use stdio stream functionality
and is useful in debugging the stdio code.