In this stage, the cost engine traverses each process-operation hierarchy. It uses post-order, depth first tree traversal, evaluating each operation’s taxonomy module. This means, essentially, that costing proceeds in a bottom-up fashion, so that all the children of a given operation are costed before that operation is costed. That way, an operation taxonomy module can access the results of the modules associated with its children in the process-operation hierarchy, in order, for example, to aggregate their cycle times.