intro k xs ys h
κ : Type u_1
ν : Type u_2
inst✝ : DecidableEq κ
⊢ ∀ (k : κ) (xs ys : List (κ × ν)), xs <+ ys → (matchKey xs k).snd <+ (matchKey ys k).snd
κ : Type u_1
ν : Type u_2
inst✝ : DecidableEq κ
k : κ
xs ys : List (κ × ν)
h : xs <+ ys
⊢ (matchKey xs k).snd <+ (matchKey ys k).snd