I just added an example of MEP source code with Automatically Defined Functions (ADFs) for solving the even parity problems:
Even parity is a function with n Boolean inputs which must return 1 if there are an even number of 1s in its inputs, otherwise returns 0.
I have added an example for the Even-6-parity function.
MEP with ADFs is able to solve larger instances of this problem compared to MEP without ADFs.
An earlier version of this code has been used in the paper:
Oltean Mihai, Improving Multi Expression Programming: an Ascending Trail from Sea-level Even-3-parity Problem to Alpine Even-18-Parity Problem, contributed chapter, Evolvable Machines: Theory and Practice, edited by Nadia Nedjah (et. al), pages 229-255, Springer-Verlag, Berlin, 2004
which can be read from here: