[Grace-core] collections interface for new classes

[James Noble]

> Not requiring the second set of brackets is quite important,

Yes...
Hopefully Michael can point out what the issue is...

> I think the issue is distinguishing between [ ] as an operator, ... and as a sequence denotation

Again yes

> Two simple solutions are to keep [ ] as an operator, and use something else (like ⟨ ⟩) for sequences, or to appropriate [ and ] for sequences and remove them from the set of available operators.

Operators have to be delineated by spaces in Kernan - which I've now got used to and like: it does makes things easier to read. So one option is that indexing operators cannot be preceded by spaces, but literals must be. 

J