`depth:`procedureDepth-first search depth; given a list of initial nodes, this procedure computes shortest DFS depth for each nodes traversed, and the number of nodes visited while traversing the successors of each node.

`NODE-DEPTH`is an array that contains the corresponding DFS depth for each node;`TRAVERSAL-TIME`is also an array, where each value is the number of nodes visited in the sub-graph of that node.