code: func (cacher *txSenderCacher) cache() {
  	for task := range cacher.tasks {
  		for i := 0; i < len(task.txs); i += task.inc {
  			types.Sender(task.signer, task.txs[i])
  		}
  	}
  }
docs: // cache is an infinite loop, caching transaction senders from various forms of
  // data structures.