We propose expression simplification and tree
compression as aids in understanding the evolution of
regular structure in Genetic Programming individuals.
We apply the analysis to two previously-published
algorithms, which aimed to promote regular and repeated
structure. One relies on subtree duplication operators,
the other uses repeated evaluation during a
developmental process. Both successfully generated
solutions to difficult problems, their success being
ascribed to promotion of regular structure. Our
analysis modifies this ascription: the evolution of
regular structure is more complex than anticipated, and
the success of the techniques may have arisen from a
combination of promotion of regularity, and other, so
far unidentified, effects.