code: func (m *txSortedMap) Put(tx *types.Transaction) {
  	nonce := tx.Nonce()
  	if m.items[nonce] == nil {
  		heap.Push(m.index, nonce)
  	}
  	m.items[nonce], m.cache = tx, nil
  }
docs: // Put inserts a new transaction into the map, also updating the map's nonce
  // index. If a transaction already exists with the same nonce, it's overwritten.