code: func TxDifference(a, b Transactions) Transactions {
  	keep := make(Transactions, 0, len(a))
  
  	remove := make(map[common.Hash]struct{})
  	for _, tx := range b {
  		remove[tx.Hash()] = struct{}{}
  	}
  
  	for _, tx := range a {
  		if _, ok := remove[tx.Hash()]; !ok {
  			keep = append(keep, tx)
  		}
  	}
  
  	return keep
  }
docs: // TxDifference returns a new set which is the difference between a and b.