code: func (m *MutexKV) get(key string) *sync.Mutex {
  	m.lock.Lock()
  	defer m.lock.Unlock()
  	mutex, ok := m.store[key]
  	if !ok {
  		mutex = &sync.Mutex{}
  		m.store[key] = mutex
  	}
  	return mutex
  }
docs: // Returns a mutex for the given key, no guarantee of its lock status