add: type assertion

interfaces/04-interfaces/puzzle.go

@@ -14,6 +14,8 @@ type puzzle struct {
 	price money
 }
 
+// if you remove this method,
+// you can no longer add it to the `store` in `main()`.
 func (p puzzle) print() {
 	fmt.Printf("%-15s: %s\n", p.title, p.price.string())
 }