def free(self):
        return self.visit(lambda e: e.free(),
                          lambda parts: reduce(operator.or_, parts, set()))
Algorithm:
dataset/ETHPy150Open nltk/nltk/nltk/sem/logic.py/Expression.free--crossover with--dataset/ETHPy150Open algorhythms/HackerRankAlgorithms/Palindrome Index.py/Solution.solve