* Creates a {@link Row} for each reachable {@link FlowNode}
*/
private Map<FlowNode, Row> createAllRows() {
heads = execution.getCurrentHeads();
FlowGraphWalker walker = new FlowGraphWalker();
walker.addHeads(heads);
// nodes that we've visited
Map<FlowNode,Row> rows = new LinkedHashMap<FlowNode, Row>();
FlowNode n;