Post #2069364

@kevin Best I can give you if that for any type you use to represent the tree, it's derivative (https://en.wikipedia.org/wiki/Generalizations_of_the_derivative#Derivative_of_a_type) is a tree plus one cursor (hole). (This generalizes zippers) If that's useful you might check out http://strictlypositive.org/CJ.pdf from @pigworker Your "jokers" are everything before/above your hole; the "clowns" are everything after/below your hole. (This captures a traversal-in-progress, "stuck in the middle" with you.)