index.js
function clear(timer){
  log('stop');
  timer.state.intervalTimer = clearInterval(timer.state.intervalTimer);
  timer.state.retryCounter.clear();
  return timer;
}
function clear(timer){
  log('stop');
  timer.state.intervalTimer = clearInterval(timer.state.intervalTimer);
  timer.state.retryCounter.clear();
  return timer;
}
a timer: a -> a