Echo the form parameter.

def form_echo(cls, request,
                  foo: (Ptypes.form, String('A form parameter'))) -> [
            (200, 'Ok', String)]:
        '''Echo the form parameter.'''
        log.info('Echoing form param, value is: {}'.format(foo))
        for i in range(randint(0, MAX_LOOP_DURATION)):
            yield
        msg = 'The value sent was: {}'.format(foo)
        Respond(200, msg)