have : m ≤ Finset.sup p.fvarList.toFinset id :=
  Finset.le_sup (s := p.fvarList.toFinset) (b := m) (f := id) (by simpa using hm)
L : Language
ξ : Type u_1
n n₁ n₂✝ n₂ m m₁ m₂ m₃ : ℕ
p : SyntacticSemiformula L n
h : (fvarList p).toFinset.sup id < m
hm : m ∈ fvarList p
⊢ False
L : Language
ξ : Type u_1
n n₁ n₂✝ n₂ m m₁ m₂ m₃ : ℕ
p : SyntacticSemiformula L n
h : (fvarList p).toFinset.sup id < m
hm : m ∈ fvarList p
this : m ≤ (fvarList p).toFinset.sup id
⊢ False