Question on tree-walking and mutually-recursive types

[Richard Kenner]

    I'm confused.
    Why can't you just use walk_tree_without_duplicates to do your tree 
    walk in the caller, instead of polluting walk_tree?

    Or is this common code that requires a walk_tree instead (it's not 
    clear), and if so, what code exactly?

It's *all* the tree walking that can run into this.  I'm seeing it for
the walk using check_pointer_types_r, but it could occur on any of them.