What is the simplest way to say "$\varphi$ is a well-formed formula of formal system $\mathbf{F}$" in symbols? The only thing that comes to mind is: $$\varphi \in \mathbf{F}$$
Am I right?
I.e., would it be acceptable to say (somewhat cumbersomely): $$'\exists x (x=x)'\in \mathbf{ZFC}$$