intros xs k
κ : Type u_1
ν : Type u_2
inst : DecidableEq κ
⊢ ∀ (xs : List (κ × ν)) (k : κ), (matchKey xs k).fst = map Prod.snd (filter (fun x => decide (x.fst = k)) xs)
κ : Type u_1
ν : Type u_2
inst : DecidableEq κ
xs : List (κ × ν)
k : κ
⊢ (matchKey xs k).fst = map Prod.snd (filter (fun x => decide (x.fst = k)) xs)