Anything involving transitive closure, for example. Say you have a directed graph, encoding an edge from `x` to `y` as `edge(x, y)` (or as a triple `(x, edge, y)`, and you want to find all vertices reachable from `a`. In standard Datalog, you'd have two rules `reachable(x, y) :- edge(x, y)` and `reachable(x, z) :- reachable(x, y), edge(y, z)`, with the second being the recursive rule.
