ledger balances helpers like Sub, Sum, Invert

cmd/cli/main.go

@@ -4,7 +4,6 @@ import (
 	"flag"
 	"fmt"
 	"io"
-	"maps"
 	"os"
 	"slices"
 
@@ -101,14 +100,7 @@ func Main() {
 		for _, date := range register.Dates() {
 			balances := register[date]
 
-			newBalances := make(ledger.Balances)
-			for k, v := range balances {
-				if got := prev[k]; !maps.Equal(v, got) {
-					newBalances[k] = v
-				}
-			}
-
-			if len(newBalances) > 0 {
+			if newBalances := balances.Sub(prev).Nonzero(); len(newBalances) > 0 {
 				fmt.Println(date)
 				FPrintBalances(os.Stdout, newBalances)
 			}