Better error messages:

Better error messages:

 - handle errors in rule initializers.

 - include rule source in error message

 - no need to run delete-updates when an item "was" causing an error because
   these updates don't land on the agenda.

REPL uses correct rule indexes

 - Parser state maintained across calls.

 - colon equals is no longer buggy in the REPL.

Codegen minor aesthetics