1 apple: red
2 book: square
3 kite: red, square
4 banana: 
5 postboxes: red

+-o tags
| +-o red
| | +-o postboxes.5
| | +-o apple.1.doc
| | +-o kite.3.doc
| | +-o square
| |   +-o kite.3.doc
| +-o square
|   +-o book.2.doc
|   +-o kite.3.doc
|   +-o red
|   | +- kite.3.doc
|   +-o -red
|     +-o book.2.doc
+-o untagged
|  +-o banana.4.doc
+-o view
  +-o "square -red"
    +-o book.2.doc
