cases a1
case intro
σ : Instantiation
σ' : VarName → VarName
a1 : ↑σ ∘ σ' = id ∧ σ' ∘ ↑σ = id
⊢ Function.Injective ↑σ
case intro.intro
σ : Instantiation
σ' : VarName → VarName
left✝ : ↑σ ∘ σ' = id
right✝ : σ' ∘ ↑σ = id
⊢ Function.Injective ↑σ