code: def simplify(self):
          if self.iscanonical:
              return self
          args = [arg.simplify() for arg in self.args]
          expr = self.__class__(*args)
          expr = expr.literalize()
          expr = expr.flatten()
          if self.annihilator in expr.args:
              return self.annihilator
          args = []
          for arg in expr.args:
              if arg not in args:
                  args.append(arg)
          if len(args) == 1:
              return args[0]
          if self.identity in args:
              args.remove(self.identity)
              if len(args) == 1:
                  return args[0]
          for arg in args:
              if self.NOT(arg) in args:
                  return self.annihilator
          i = 0
          while i < len(args) - 1:
              j = i + 1
              ai = args[i]
              if not isinstance(ai, self.dual):
                  i += 1
                  continue
              while j < len(args):
                  aj = args[j]
                  if not isinstance(aj, self.dual) or len(ai.args) != len(aj.args):
                      j += 1
                      continue
                  negated = None
                  for arg in ai.args:
                      if arg in aj.args:
                          pass
                      elif self.NOT(arg).cancel() in aj.args:
                          if negated is None:
                              negated = arg
                          else:
                              negated = None
                              break
                      else:
                          negated = None
                          break
                  if negated is not None:
                      del args[j]
                      aiargs = list(ai.args)
                      aiargs.remove(negated)
                      if len(aiargs) == 1:
                          args[i] = aiargs[0]
                      else:
                          args[i] = self.dual(*aiargs)
                      if len(args) == 1:
                          return args[0]
                      else:
                          return self.__class__(*args).simplify()
                  j += 1
              i += 1
          args = self.absorb(args)
          if len(args) == 1:
              return args[0]
          args.sort()
          expr = self.__class__(*args)
          expr.iscanonical = True
          return expr