intro psi_in_insert
ϕ ψ : Formula
S : Finset Formula
a✝¹ : ψ ∉ S
a✝ : ϕ ∈ S → vocabOfFormula ϕ ⊆ vocabOfSetFormula S
⊢ ϕ ∈ insert ψ S → vocabOfFormula ϕ ⊆ vocabOfSetFormula (insert ψ S)
ϕ ψ : Formula
S : Finset Formula
a✝¹ : ψ ∉ S
a✝ : ϕ ∈ S → vocabOfFormula ϕ ⊆ vocabOfSetFormula S
psi_in_insert : ϕ ∈ insert ψ S
⊢ vocabOfFormula ϕ ⊆ vocabOfSetFormula (insert ψ S)