def simple_app(environ, start_response, extra_headers=None):
    """Simple WSGI application"""
    data = 'Hello world!\n'
    response_headers = [('Content-type', 'text/plain'),
                        ('Content-length', '%s' % len(data))]
    if extra_headers:
        response_headers += extra_headers
    start_response('200 OK', response_headers)
    return [data]