Write a python function most_popular_friend(d) that takes in a dictionary that maps names to a set of the names of their friends and returns the most popular friend. In other words, it returns the name that occurs the most often in the sets of friends. You may assume that ties will not occur.

Test cases

d0 = { "john": {"joy", "jim", "jade"},

"jade": {"joy", "rose", "john"}}

most_popular_friend(d0) == "joy"

d1 = {"A": {"B"},

"B": {"C", "D", "E", "A"},

"C": {"A", "D"},

"D": {},

"E": {"A", "B", "C"}}