I am  pretty new to evolutionary Computation and thus not experienced. So
forgive me, if my questions are dull..

I am trying to set up a GP-System that can create a PID-controller
Thus for doing that, I need 
a) a node, that can sum up the value another node holds and
b) a node, that contains the value another node had in the cycle before.

The solution my GP System should find is:

y = Kp * e + Ki * Ta * esum + Kd * (e  ealt)/Ta

where Kp, Ki, Ta, Kd are constants
and e is calculated out of values changing each timestep. ( so practically
"e" isn't a terminal symbol, but rather a subtree which result is the value
of "e".

so, plainly speaking I have a subtree which returns the value of "e". I then
need a node "e_sum" and a node "e_old".

Any hints on how to implement those nodes? Do you need additional information?

Best Regards,

Thorsten Tiede