For example, Nationality(x, y) ∧ Nationality(z, y) ∧ Language(z, w) ⇒ Language(x, w) is a high-quality formula, which means people with the same nationality probably speak the same language.
it is a challenge to create formulas for open-domain KBs, where there are a great variety of relation types and it is impossible to construct a complete formula set by hand.