Define a function breadthFirst, which performs a breadth-first traversal on a graph, given a start vertex. This function should return a list of the labels of the vertices in the order in which they...

Define a function breadthFirst, which performs a breadth-first traversal on a graph, given a start vertex. This function should return a list of the labels of the vertices in the order in which they are visited. Test the function thoroughly with the case study program.