code: func (c *Collection) Put(i gtreap.Item) {
  	c.assertNotReadOnly()
  
  	// Lock around the entire Upsert operation to serialize Puts.
  	priority := rand.Int()
  	c.rootLock.Lock()
  	c.root = c.root.Upsert(i, priority)
  	c.rootLock.Unlock()
  }
docs: // Put adds an item to the Store.
  //
  // If the Store is read-only, Put will panic.