The type of variables, or graph vertices.

foreach_root describes the root nodes of the data flow graph as well as the properties associated with them. foreach_root contribute must call contribute x p to indicate that x is a root and that p is a lower bound on the solution at x. It may call contribute x _ several times at a single root x.

foreach_successor describes the edges of the data flow graph as well as the manner in which a property at the source of an edge is transformed into a property at the target. The property at the target must of course be a monotonic function of the property at the source.