case not_ phi phi_ih =>
  simp only [sub]
  unfold Instantiation.comp
  congr! 1
σ σ' : Instantiation
phi : Formula
phi_ih : sub σ meta_var_ (sub σ' meta_var_ phi) = sub (σ.comp σ') meta_var_ phi
⊢ sub σ meta_var_ (sub σ' meta_var_ phi.not_) = sub (σ.comp σ') meta_var_ phi.not_
no goals