have := Nat.lt_irrefl (freshVarForm φ)
φ : Formula
hyp : freshVarForm φ ∈ vocabOfFormula φ
this : freshVarForm φ ∈ vocabOfFormula φ → freshVarForm φ < freshVarForm φ
⊢ False
φ : Formula
hyp : freshVarForm φ ∈ vocabOfFormula φ
this✝ : freshVarForm φ ∈ vocabOfFormula φ → freshVarForm φ < freshVarForm φ
this : ¬freshVarForm φ < freshVarForm φ
⊢ False