max_by =: 1 : '{~ (i. >./)@:(u&>)'
For example the list.max_by(&:length) is (# max_by) Example: Sally has three brothers, Sally and their brothers have the same mother but a different father, and those brothers have two sisters Sally and Mary, but Mary and Sally are not sisters because they are from different fathers and mothers, hence Sally has no sister.
For those mathematically inclined: Supposing the three brothers are called Bob (to simplify) and the parents are designed by numbers.