Before revision: A TGD is a sentence in first-order logic of the form: ∀x1 ... xn, P(x1, ..., xn) → ∃y1, ..., ym, Q(x1, ..., xn,y1, ..., ym), where P is a possibly empty and Q is a non-empty conjunction of relational atoms.
After revision: A TGD is a sentence in first-order logic of the form: , where is a possibly empty and is a non-empty conjunction of relational atoms.