1. A computer implemented method comprising: given a current node and an arc pointing from the current node to a next node, analyzing arcs in a data structure to determine which of the arcs are valid arcs pointing from the next node; constructing arc configuration information associated with the next node, the arc configuration information representing the valid arcs pointing from the next node; and storing the arc configuration information associated with the next node, enabling the arc configuration information to be evaluated and the valid arcs pointing from the next node to be identified from the evaluation of the arc configuration information without the next node being read.