⊦ reach<(a, b, n) ↔ (∃ m. m < n ∧ reach(a, b, m))
∃ m. m < n + 1 ∧ reach(a, b, m) ⊦ ∃ m. m < n + 2 ∧ reach(a, b, m)
[[CONCRETESIMPRULES : ()], [EXTRATERMS : (n + 1, b, a)]]