lets do composition in dart without inheritance. we have public api with classes E, U and no more. now there are components Merch with buy(), firstMerch with component Merch with buy() forwarded with some logic, and secondMerch with component Merch  with buy() forwarded but without extra logic. We have composite classes U with component Singlemerch and forwarded buy() with some further logic, class E with component Multimerch and forwarding buy with its own logic. show this in web code block 