NestedSet's are a great way of using nested data in a programmatic way. However working with them is quite complex without any helpers. Doctrine provides a NestedSet manager which is built-in into symfony and Diem. While the Diem 5.1-nestedset branch handles nestedsets in the admin, a ready to use nested set is not available in the front widgets.
As this is quiet straightforward and a common feature I'll stow how I usually render a nestedset in a typical category/products relationship.