Provide support for negative indexes as end relative offsets

[hopscc]

Discussion at
 Negative indexes as end offsets

Provide an extended indexing construct [[ ]] analogous to [ ]
Extended indexes support both positive and negative indices.
Positive indexes are as for [] indexing.
Negative indexes are treated as relative to the end of the collection with

coll[[-1]]

accessing the last item of the collection coll.

Dont affect performance of [] indexing.
Both positive and negative indexes off the edges of the collection throw an exception as per [] indexing.

[hopscc]

extended Index patch with fixed line endings

[Chuck]

As discussed in the forum, I don't like the

Error: Failed to load processor i

No macro or processor named 'i' found

syntax. My current plan is still .last and .at.