private int getNumber(final TreeNode node, final TreeNode parentNode) {
        Integer n = number.get(node);
        if (n == null) {
            int i = 1;
            for (final TreeNode child : tree.getChildren(parentNode)) {
                number.put(child, i++);
            }
            n = number.get(node);
        }

        return n.intValue();
    }
@param node       [tree.isChildOfParent(node, parentNode)]
@param parentNode parent of node
@return
stroom-core-client-widget/src/main/java/stroom/widget/htree/client/treelayout/AbegoTreeLayout.java