rcases a_in with ⟨θ, _, a_in_vocTheta⟩
case a.mp
X : Finset Formula
ψ ϕ : Formula
psi_in_X : ψ ∈ X
eq_voc : vocabOfFormula ϕ = vocabOfFormula ψ
a✝ : Char
a_in : ∃ a ∈ X, a✝ ∈ vocabOfFormula a
⊢ a✝ ∈ vocabOfFormula ϕ ∨ ∃ a, (¬a = ψ ∧ a ∈ X) ∧ a✝ ∈ vocabOfFormula a
case a.mp.intro.intro
X : Finset Formula
ψ ϕ : Formula
psi_in_X : ψ ∈ X
eq_voc : vocabOfFormula ϕ = vocabOfFormula ψ
a✝ : Char
θ : Formula
left✝ : θ ∈ X
a_in_vocTheta : a✝ ∈ vocabOfFormula θ
⊢ a✝ ∈ vocabOfFormula ϕ ∨ ∃ a, (¬a = ψ ∧ a ∈ X) ∧ a✝ ∈ vocabOfFormula a