simp only [ncols, Schema.length_eq_List_length]
η : Type u_η
dec_η : DecidableEq η
sch : Schema
t : Table sch
⊢ ncols t = List.length (header t)
η : Type u_η
dec_η : DecidableEq η
sch : Schema
t : Table sch
⊢ List.length sch = List.length (header t)