rcases j with ⟨j,hj⟩
case mk
α : Type u
x : Subarray α
j : Fin (size x)
i : Nat
hi : i < size x
⊢ (swap x { val := i, isLt := hi } j)[↑{ val := i, isLt := hi }] = x[↑j]
case mk.mk
α : Type u
x : Subarray α
i : Nat
hi : i < size x
j : Nat
hj : j < size x
⊢ (swap x { val := i, isLt := hi } { val := j, isLt := hj })[↑{ val := i, isLt := hi }] = x[↑{ val := j, isLt := hj }]