item of shelf indexer?
When it is known that a shelf has a specific key, it is possible to find which item has that key with the
item of operator.
This operation gives the position of the specified item on the shelf. It is an error if the specified item does not exist. One way to avoid this error when selecting an item by its key is to guarantee that the shelf has the key before looking for its position (the
has key test can be used to do this).