simp only [getColumn1, nrows, Schema.length_eq_List_length]
η : Type u_η
dec_η : DecidableEq η
sch : Schema
t : Table sch
n : Nat
h : n < ncols t
⊢ List.length (getColumn1 t n) = nrows t
η : Type u_η
dec_η : DecidableEq η
sch : Schema
t : Table sch
n : Nat
h : n < ncols t
⊢ List.length (List.map (fun r => Cell.toOption (Row.nth r n h)) t.rows) = List.length t.rows