apply Mem.tail
case tail
κ : Type u_1
ν : Type u_2
inst✝ : DecidableEq κ
xs✝ ys✝ xs ys : List (κ × ν)
k : κ
v : ν
hsubl : xs <+ ys
x : κ
htail : Mem x (map Prod.fst (groupByKey (matchKey xs k).snd))
⊢ x ∈ k :: map Prod.fst (groupByKey (matchKey ys k).snd)
case tail.a
κ : Type u_1
ν : Type u_2
inst✝ : DecidableEq κ
xs✝ ys✝ xs ys : List (κ × ν)
k : κ
v : ν
hsubl : xs <+ ys
x : κ
htail : Mem x (map Prod.fst (groupByKey (matchKey xs k).snd))
⊢ Mem x (map Prod.fst (groupByKey (matchKey ys k).snd))