code: func (m *txSortedMap) Forward(threshold uint64) types.Transactions {
  	var removed types.Transactions
  
  	// Pop off heap items until the threshold is reached
  	for m.index.Len() > 0 && (*m.index)[0] < threshold {
  		nonce := heap.Pop(m.index).(uint64)
  		removed = append(removed, m.items[nonce])
  		delete(m.items, nonce)
  	}
  	// If we had a cached order, shift the front
  	if m.cache != nil {
  		m.cache = m.cache[len(removed):]
  	}
  	return removed
  }
docs: // Forward removes all transactions from the map with a nonce lower than the
  // provided threshold. Every removed transaction is returned for any post-removal
  // maintenance.