code: def collapse_nested(self, cats, max_nestedness=10):
          
          children = []
          removed = set()
          nestedness = max_nestedness
  
          old = list(self.widget.options.values())
          nested = [cat for cat in old if getattr(cat, 'cat') is not None]
          parents = {cat.cat for cat in nested}
          parents_to_remove = cats
          while len(parents_to_remove) > 0 and nestedness > 0:
              for cat in nested:
                  if cat.cat in parents_to_remove:
                      children.append(cat)
              removed = removed.union(parents_to_remove)
              nested = [cat for cat in nested if cat not in children]
              parents_to_remove = {c for c in children if c in parents - removed}
              nestedness -= 1
          self.remove(children)
docs: Collapse any items that are nested under cats.
          `max_nestedness` acts as a fail-safe to prevent infinite looping.