I disagree. I would call this a proof but not a derivation. In other words, each step follows logically from the previous ones, even if there's no way that you

I disagree. I would call this a proof but not a derivation. In other words, each step follows logically from the previous ones, even if there's no way that you would discover the final equations this way if you didn't already know or guess them. In general, this is part of what makes reading mathematical papers so difficult; the steps of the proofs are almost never written down in the order they were discovered. All of the false starts and a lot of the intuition is discarded to yield a shorter but sometimes totally mysterious path from the assumptions to the conclusion.