implement transaction.go:Transactions:expenses to return a new slice of only the expense transactions

digits-work-sample-go.d/transaction.go

@@ -50,6 +50,16 @@ func (trn Transaction) isRefund() bool {
 // Transactions represents a list of Transaction
 type Transactions []Transaction
 
+func (trns Transactions) expenses() Transactions {
+	result := make(Transactions, 0, len(trns))
+	for i := range trns {
+		if !trns[i].isRefund() {
+			result = append(result, trns[i])
+		}
+	}
+	return result
+}
+
 // Sum adds all transactions together.
 func (trns Transactions) Sum() Amount {
 	result := Amount(0.0)