simp only [groupByKey]
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
hx : x ∈ map Prod.fst (groupByKey ((k, v) :: xs))
⊢ x ∈ map Prod.fst (groupByKey ((k, v) :: ys))
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
hx : x ∈ map Prod.fst (groupByKey ((k, v) :: xs))
⊢ x ∈ map Prod.fst ((k, v :: (matchKey ys k).fst) :: groupByKey (matchKey ys k).snd)