code: func (l *ItemList) insert(e, at *ItemElement) *ItemElement {
  	n := at.next
  	at.next = e
  	e.prev = at
  	e.next = n
  	n.prev = e
  	e.list = l
  	l.len++
  	return e
  }
docs: // insert inserts e after at, increments l.len, and returns e.