import fi.uef.cs.tra.DiGraph;
import fi.uef.cs.tra.Vertex;
import java.util.Set;

public interface TRAII_25_X4 {

    /**
     * All the vertices that do not have incoming edges.
     * @param G input graph
     * @return set of root vertices
     */
    Set<Vertex> rootVertics(DiGraph G);

    /**
     * How many trees there are in the graph?
     * A tree has no back-, cross- or forward edges.
     * @param G input graph
     * @return number of trees
     */
    int treeCount(DiGraph G);
}