Just a heads-up to the author: >Ancestor(X, Y) → Parent(X, Y) ∨ (Parent(X, Z) ∧ Ancestor(Z, Y)). That's a bug. The arrow needs to go to the left. Also, then,

Just a heads-up to the author: >Ancestor(X, Y) → Parent(X, Y) ∨ (Parent(X, Z) ∧ Ancestor(Z, Y)). That's a bug. The arrow needs to go to the left. Also, then, you can also opt to dispense with the disjunction and just use first-order logic normally. Ancestor(X, Y) ← Parent(X, Y). Ancestor(X, Y) ← Parent(X, Z) ∧ Ancestor(Z, Y).