alias state in der-call

[Philip Hannebohm]

Currently arguments of the der-call like in der(sin(x)*y) = 1 are differentiated symbolically until single variables are reached, yielding something like

cos(x)*der(x)*y + sin(x)*der(y) = 1

In cases like der(y^x) this may even result in pretty ugly residual equations.

Instead we could introduce an alias z = sin(x)*y, s.t. der(z) = 1. I believe Karim has some convincing arguments towards this approach. I'll try to recap them from what I understand:

1. If x and y appear only algebraically elsewhere, introducing an alias results in fewer states, thus a lower index.
2. Otherwise (der(x) and der(y) exist elsewhere) the system may need index reduction anyways, because the states x and y are connected somehow. The alias might allow index reduction to pick a better choice.

I personally think differentiating the interior of der(...) is a cool feature. But there may be practical reasons against it.
I have no idea how academic this issue is. Opinions?