apply List.length_map
η : Type u_η
dec_η : DecidableEq η
sch : Schema
t✝ : Table sch
c✝ : η
hc✝ : Schema.HasName c✝ sch
⊢ List.length (List.map (Row.removeColumn hc✝) t✝.rows) = List.length t✝.rows
no goals