apply Eq.symm
η : Type u_η
dec_η : DecidableEq η
sch : Schema
t : Table sch
⊢ List.length sch = List.length (header t)
case h
η : Type u_η
dec_η : DecidableEq η
sch : Schema
t : Table sch
⊢ List.length (header t) = List.length sch