diff --git a/src/__tests__/money.test.ts b/src/__tests__/money.test.ts
index d794737..9889a10 100644
--- a/src/__tests__/money.test.ts
+++ b/src/__tests__/money.test.ts
@@ -1,7 +1,7 @@
 // import { Dollar } from '../dollar';
 // import { Franc } from '../franc';
-import { Bank } from '../bank';
 import { Money } from '../money';
+import { Bank } from '../bank';
 
 test('equals', () => {
   expect(Money.dollar(5).equals(Money.dollar(5))).toBeTruthy();
@@ -34,7 +34,7 @@ test('times', () => {
 });
 
 test('simple addition', () => {
-  const sum :Money = Money.dollar(5).plus(Money.dollar(5));
+  const sum = Money.dollar(5).plus(Money.dollar(5));
   expect(sum).toEqual(Money.dollar(10));
 
   const bank = new Bank();
diff --git a/src/bank.ts b/src/bank.ts
index aa52ce7..7b32108 100644
--- a/src/bank.ts
+++ b/src/bank.ts
@@ -1,7 +1,8 @@
+import { Expression } from './expression';
 import { Money } from './money';
 
 export class Bank {
-  reduce(sum: Money, currency: string):Money {
+  reduce(sum: Expression, currency: string):Money {
     return Money.dollar(10);
   }
 }
diff --git a/src/expression.ts b/src/expression.ts
new file mode 100644
index 0000000..6640bef
--- /dev/null
+++ b/src/expression.ts
@@ -0,0 +1,2 @@
+export interface Expression{
+}
diff --git a/src/money.ts b/src/money.ts
index 442019b..e735f36 100644
--- a/src/money.ts
+++ b/src/money.ts
@@ -1,4 +1,6 @@
-export class Money {
+import { Expression } from "./expression";
+
+export class Money implements Expression {
   constructor(protected readonly amount: number, public readonly currency: string) {
   }
 
@@ -26,7 +28,7 @@ export class Money {
     return `${this.amount} ${this.currency}`
   }
 
-  plus(addend: Money):Money {
+  plus(addend: Money):Expression {
     return new Money(this.amount + addend.amount, this.currency)
   }
 }