Fair enough, I misled myself.

However, and I feel like I'm getting really picky here, but it still doesn't fulfill the paragraph I quoted:

    def application(environ, start_response):
        start_response('200 OK',
                   [('Content-type', 'text/plain')])
        yield b''

        try:
            # produce an exception tuple, so we can re-call s_r
            raise RuntimeError
        except RuntimeError:
            # Headers shouldn't have been sent, but they were
            # so this will throw:
            start_response('200 OK',
                           [('Content-type', 'text/plain')],
                           sys.exc_info())
        yield b'error data or whatever'

But if async support a foregone conclusion anyway, is it worth bothering complying with that odd requirement?