new (and successful :) ) story for 'parent/overridden irtrees'. basically, if we have a rule which maps some eval X's type from A -> B, we preserve the original ir-tree compilation of A in the 'history' when we inevitably produce the ir-tree compilation of B. this can be used for accesses on overridden evaluations, and it allows us to implement $ in auto view rules.

src/eval/eval_core.h

@@ -271,6 +271,7 @@ struct E_IRTreeAndType
   void *user_data;
   E_Mode mode;
   E_MsgList msgs;
+  E_IRTreeAndType *prev;
 };
 
 ////////////////////////////////