Theorem: LO.FirstOrder.Derivation.length_cast' (Logic/FirstOrder/Basic/Calculus.lean)

State before:
L : Language
Δ Δ₁ Δ₂ Γ : Sequent L
p q r : SyntacticFormula L
d :  ⊢¹ Δ
e : Δ = Γ
⊢ length (cast ⋯ d) = length d
Tactic:
rcases e with rfl
State after:
L : Language
Δ Δ₁ Δ₂ : Sequent L
p q r : SyntacticFormula L
d :  ⊢¹ Δ
⊢ length (cast ⋯ d) = length d

State before:
L : Language
Δ Δ₁ Δ₂ : Sequent L
p q r : SyntacticFormula L
d :  ⊢¹ Δ
⊢ length (cast ⋯ d) = length d
Tactic:
simp[Derivation.cast]
State after:
no goals