control structure
using referents indexer
The using referents prefix sets a default item on the referents shelf.
Indexer is an index phrase, giving either the key or the index of the referent to use.
References to the referent selected by a using referents prefix are different from those involving other shelves such as integers or attributes. With other shelves, the name of a shelf inside the body of the using prefix refers to the selected item. But the referents shelf does not have a name. The selected referent is referred to by the keywords this referent.