⊦ reachable(r, r0, H) ↔ (∃ x. path(r ' + x, H) ∧ (r ' + x).last = r0)
H[r].nxt = null, r0 ∈ H, ∃ x. path(r ' + x, H) ∧ (r ' + x).last = r0 ⊦ r = r0, r0 = null
Used terms: H, r0, r