have ih := length_groupByKey (matchKey xs k).2
κ : Type u_1
inst✝ : DecidableEq κ
ν : Type u_2
k : κ
v : ν
xs : List (κ × ν)
hterm : length (matchKey xs k).snd < Nat.succ (length xs)
⊢ length (groupByKey ((k, v) :: xs)) = length (unique (map Prod.fst ((k, v) :: xs)))
κ : Type u_1
inst✝ : DecidableEq κ
ν : Type u_2
k : κ
v : ν
xs : List (κ × ν)
hterm : length (matchKey xs k).snd < Nat.succ (length xs)
ih : length (groupByKey (matchKey xs k).snd) = length (unique (map Prod.fst (matchKey xs k).snd))
⊢ length (groupByKey ((k, v) :: xs)) = length (unique (map Prod.fst ((k, v) :: xs)))