case union α β =>
  have IHα := substitutionLemmaRel σ α M
  have IHβ := substitutionLemmaRel σ β M
  simp_all only [subst_in_P, relate]
σ : Substitution
W : Type
M : KripkeModel W
w v : W
α β : Program
⊢ relate M (subst_in_P σ (α⋓β)) w v ↔ relate (subst_in_model σ M) (α⋓β) w v
no goals