code: def expand_nested(self, cats):
          
          down = '│'
          right = '└──'
  
          def get_children(parent):
              return [e() for e in parent._entries.values() if e._container == 'catalog']
  
          if len(cats) == 0:
              return
  
          cat = cats[0]
          old = list(self.options.items())
          name = next(k for k, v in old if v == cat)
          index = next(i for i, (k, v) in enumerate(old) if v == cat)
          if right in name:
              prefix = f'{name.split(right)[0]}{down} {right}'
          else:
              prefix = right
  
          children = get_children(cat)
          for i, child in enumerate(children):
              old.insert(index+i+1, (f'{prefix} {child.name}', child))
          self.widget.options = dict(old)
docs: Populate widget with nested catalogs