Design and Retrieve Items in a Nested Box

Design a shopping box system with a nested structure, where each box can contain various items (e.g., toothbrush) or other boxes. Implement a function to return all concrete items inside a given shopping box. Example Input {"boxes": [{"name": "box1", "items": ["toothbrush"]}, {"name": "box2", "items": ["box1", "soap"]}]