code: def push(self, cf):
          
          cf.next = self
          if self.state is not None:
              self.state.register_plugin('callstack', cf)
              self.state.history.recent_stack_actions.append(CallStackAction(
                  hash(cf), len(cf), 'push', callframe=cf.copy({}, with_tail=False)
              ))
  
          return cf
docs: Push the frame cf onto the stack. Return the new stack.