rcases j with ⟨j,hj⟩
case mpr.mk
α : Type u
n : Nat
inst✝ : DecidableEq α
x : SizedArray α n
h : ∀ (i j : Fin n), x[↑i] = x[↑j] → i = j
j : Fin (Array.size x.val)
i : Nat
hi : i < Array.size x.val
hxij : x.val[↑{ val := i, isLt := hi }] = x.val[↑j]
⊢ { val := i, isLt := hi } = j
case mpr.mk.mk
α : Type u
n : Nat
inst✝ : DecidableEq α
x : SizedArray α n
h : ∀ (i j : Fin n), x[↑i] = x[↑j] → i = j
i : Nat
hi : i < Array.size x.val
j : Nat
hj : j < Array.size x.val
hxij : x.val[↑{ val := i, isLt := hi }] = x.val[↑{ val := j, isLt := hj }]
⊢ { val := i, isLt := hi } = { val := j, isLt := hj }