Context

According to Wikipedia : "The seventh son of a seventh son is a concept from folklore regarding special powers given to, or held by, such a son. The seventh son must come from an unbroken line with no female siblings born between, and be, in turn, born to such a seventh son."

Your task

You will be given a string of JSON, consisting of a family tree containing people's names, genders and children. Your task will be to find the seventh sons of seventh sons in the family tree, and return a set of their names. If there are none, return an empty set.

Tips

• Have a good look at the sample test cases.

• For a seventh son to be a seventh son, there must not be any daughters in the line leading to him. There may be daughters after him, though.

• Use print_tree preloaded function to print the family tree in a compact human-readable form

• You may want to use the json module for this one.