Critical path and the arcs connecting critical
activities with non-critical activities are deleted from
early tree. The sub-graph of early tree consists of many
non-connected sub-trees. The set of all sub-trees is
denoted as ST . Fig.2 shows all sub-trees in Fig.1(b).
Deleting critical activities from all paths in PET .
Once a path set becomes empty, it is removed from PET .
The remaining paths, denoted as PST , construct all subtrees
ST