Path validator for shortest path algorithms. Shortest path algorithms typically maintain a set of partial paths
which are iteratively extended until some stopping criterion is met, e.g. the target vertex is reached. A path
validator can be used to enforce additional criteria as to the validity of a given path. A partial path can only be
extended by another edge if the path validator deems this extension feasible. As per example, a path validator can be
used to enforce that a path connecting a source to a terminal vertex visits at least $n$ vertices.