apply List.length_map
η : Type u_η
dec_η : DecidableEq η
sch : Schema
τ₁ τ₂ : Type u_1
t : Table sch
c : η
f : Option τ₁ → Option τ₂
hc : Schema.HasCol (c, τ₁) sch
⊢ List.length (List.map (fun r => Row.retypeCell r hc (Cell.fromOption (f (getValue r c)))) t.rows) = List.length t.rows
no goals