This paper presents a genotype-level distance metric for Genetic Programming (GP) based on the  symmetric difference  concept: first, the  information  contained in individuals is expressed as a set of symbols (the content of each node, its position inside the tree, and recurring parent-child structures); then, the difference between two individuals is computed considering the number of elements belonging to one, but not both, of their symbol sets.
['Marco Gaudesi', 'Giovanni Squillero', 'Alberto Paolo Tonda']
Universal information distance for genetic programming