intro rs
case nil
α : Type u_1
g : α → Bool
⊢ ∀ (rs : List α), length (filterTR.loop g [] rs) ≤ length [] + length rs
case nil
α : Type u_1
g : α → Bool
rs : List α
⊢ length (filterTR.loop g [] rs) ≤ length [] + length rs