[Grace-core] "Classical" Inheritance for Grace

[James Noble]

> The engineering tradeoffs are about encouraging programmers to be explicit about the object invariants.

right.

> And yes, the code had to do stuff conditional on whether size was nil or not.    Sometimes one has to do stuff like that.

indeed one does.  But the hope is that people can just do the most straightforward things
straightforwardly most of the time.   So yes parser combinators can be made to work with proxies, or by reflexive backpatching.  But it's better if the (relatively) straightforward code  works. 

James