Section 5, Lesson 20: Why not just use the simplified invariant?

[antoniopgs]

Pat says:
"It's hard to write an assert for x * y = k"

But is it?
Why not just do assert(tSwap.balanceOf(x) * tSwap.balanceOf(y) == k)?

Where:
k = pool initialBalance of x * pool initialBalance of y

[TilakMaddy]

Hmm nope! I actually had the same question but this is the conclusion I have come to #49