Push the frame cf onto the stack. Return the new stack.

def push(self, cf):
        """
        Push the frame cf onto the stack. Return the new stack.
        """
        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