simp[Rew.comp_app]
L : Language
Δ Δ₁ Δ₂ Γ : Sequent L
p✝ q r : SyntacticFormula L
m : ℕ
p : SyntacticSemiformula L 1
h : ¬fvar? p m
x : ℕ
hx : fvar? p x
⊢ (⇑((Rew.rewriteMap fun x => if x = m then 0 else x + 1).comp [→ &m]) ∘ Semiterm.fvar) x =
    (⇑Rew.free ∘ Semiterm.fvar) x
L : Language
Δ Δ₁ Δ₂ Γ : Sequent L
p✝ q r : SyntacticFormula L
m : ℕ
p : SyntacticSemiformula L 1
h : ¬fvar? p m
x : ℕ
hx : fvar? p x
⊢ ¬x = m