code: def retract(self, sentence):
          "Remove the sentence's clauses from the KB."
          for c in conjuncts(to_cnf(sentence)):
              if c in self.clauses:
                  self.clauses.remove(cf retract(self, sentence):
          "Remove the sentence's clauses from the KB."
          for c in conjuncts(to_cnf(sentence)):
              if c in self.clauses:
                  self.clauses.remove(c)
docs: Remove the sentence's clauses from the KB.