Given a directed graph G (V, E, L) with V the node set, E the edge set and L a function that assigns to each edge e in E a label L(e). A label constrained reachability query Q(s,t,M) tests if there exists a path from a source s to a target t with a path, which consists of edges having a label from a label set M. Give an algorithm (pseudo-code) to answer query Q. [*A straightforward way is to revise BFS or DFS traversal)