exception Cycle of variableCycle v is raised by the occurs check.
new_occurs_check is_young initiates a cycle detection phase. It produces a function check that can be applied to a number of roots. The function check visits the vertices that are reachable from some root and that satisfy the user-supplied predicate is_young. If a cycle is detected, then Cycle v is raised, where v is a vertex that participates in the cycle.
The function check has internal state and records the vertices that it has visited, so no vertex is visited twice: the complexity of the occurs check is linear in the size of the graph fragment that is visited.