Indexable

Trait for types that map indices to elements.

The mapping from indices (Idx) to elements (Elm) can be partial or total. The mapping can be accessed by get.

If all indices might not be defined (e.g. only a range of integers is valid) then then Aef should include an error like OutOfBounds or KeyNotFound.

Note that the set of defined indices can be infinite which does not naturally allow iteration. An integer map with a default element would always contain all integers indices. This implies that iteration would go through all integers.