Returns the last position of a value, or -1 if the value is not found.
Returns the position of the last of equivalent values.
Equivalence is defined by the equality operator intrinsic to the collection,
either the one given as the equals argument to its constructor, or
Object.equals by default.
For a SortedSet, this operation is fast, because it is backed by a binary
search tree.
For a SortedArray, or SortedArraySet, this is also fast, employing a binary
search.
The implementation of indexOf for Array, List, and Deque is slower
but more flexible.