As usual, just asking the question helped me find the answer by myself. This syntax works: <!--#tree expr="PARENTS[-1]" -->

I want to make a tree for navigation that always starts from the topmost object regardless of the level of the current object.

But what is the ID of the topmost object?

I tried <!--#tree "PARENTS[-1]" -->, but that gets an error.

I tried enclosing the tree in <!--#with "PARENTS[-1]"--> but that doesn't work either.