code: def replace_activities(self):
          logger.debug('Running PySB Preassembler replace activities')
          new_stmts = []
          def has_agent_activity(stmt):
              for agent in stmt.agent_list():
                  if isinstance(agent, Agent) and agent.activity is not None:
                      return True
              return False
          self._gather_active_forms()
          for j, stmt in enumerate(self.statements):
              logger.debug('%d/%d %s' % (j + 1, len(self.statements), stmt))
              if not has_agent_activity(stmt):
                  new_stmts.append(stmt)
                  continue
              stmt_agents = stmt.agent_list()
              num_agents = len(stmt_agents)
              agent_forms = [[] for a in stmt_agents]
              for i, agent in enumerate(stmt_agents):
                  if agent is not None and isinstance(agent, Agent) and \
                          agent.activity is not None:
                      base_agent = self.agent_set.get_create_base_agent(agent)
                      if agent.activity.is_active:
                          active_forms = base_agent.active_forms
                          if not active_forms:
                              active_forms = [agent]
                      else:
                          active_forms = base_agent.inactive_forms
                          if not active_forms:
                              active_forms = [agent]
                      for af in active_forms:
                          new_agent = fast_deepcopy(agent)
                          self._set_agent_context(af, new_agent)
                          agent_forms[i].append(new_agent)
                  else:
                      agent_forms[i].append(agent)
              agent_combs = itertools.product(*agent_forms)
              for agent_comb in agent_combs:
                  new_stmt = fast_deepcopy(stmt)
                  new_stmt.set_agent_list(agent_comb)
                  new_stmts.append(new_stmt)
          self.statements = new_stmts