    "Given a stream of items, produce a stream of entries.
     For each non-null item produced by the given stream
     of items, the resulting stream contains an `Entry`
     of form `key->item` where `key` is the index at which
     the item occurs in the stream of items."
    see (`value Iterable.indexed`)
    shared {<Integer->Element&Object>*} entries<Element>
                ({Element*} elements)
                        => elements.indexed;