I have: A = {a, b, c, d, e}

I want to write: B = {a, b, c, d, e, f, g}

But I don't want to write duplicate code

So I write:

   B = B(A) = {A, e, f, g} 

aka I use inheritance.

Are there easier ways? No. Inheritance is the most fundamental way of doing this. Composition is just a work around as it results in arbitrary nesting. But ultimately it's the same thing too.