code: def add_system(self, system):
          '''
          Add system to the world.
          All systems will be processed on World.process()
          system is of type System
          '''
          if system not in self._systems:
              system.set_world(self)
              self._systems.append(system)
          else:
              raise DuplicateSystemError(systemf add_system(self, system):
          '''
          Add system to the world.
          All systems will be processed on World.process()
          system is of type System
          '''
          if system not in self._systems:
              system.set_world(self)
              self._systems.append(system)
          else:
              raise DuplicateSystemError(system)
docs: Add system to the world.
          All systems will be processed on World.process()
          system is of type System