intro ℓ ℓ_in_proj
φ : Formula
R L : Finset Formula
valid_proj : ~(□φ) ∈ R
⊢ jvoc (diamondProjectTNode (Sum.inr φ) (L, R)) ⊆ jvoc (L, R)
φ : Formula
R L : Finset Formula
valid_proj : ~(□φ) ∈ R
ℓ : Char
ℓ_in_proj : ℓ ∈ jvoc (diamondProjectTNode (Sum.inr φ) (L, R))
⊢ ℓ ∈ jvoc (L, R)