[evlan-devel] Updates to language description

[Kenton V. <tem...@ga...>]

I made more changes to the language description:
- Added exceptions
- Changed upcasting method.  Now uses "switch" expression.  Note that "try" 
and "switch" use basically the same syntax.
- Added concept of "closed" classes.  A closed class is one which has a 
given set of subclasses and/or implementations, but which cannot be 
extended or implemented further.  This is useful for, say, defining Boolean 
to be only "true" or "false" and nothing else.

My writing seems to be even more confusing than usual today.  Hopefully the 
changes make sense.

http://www.evlan.org/docs/informal/values.xml
http://www.evlan.org/docs/informal/types.xml

I'm going to do some actual coding now.  :)

-Kenton Varda