term model panic

[robdockins]

Running the following script yields a panic.

enable_experimental;

f <- fresh_symbolic "f" {| [100][8] |};
a <- normalize_term {{ \n -> f @ n }};

%< ---------------------------------------------------
  Revision:  abd6f202ccc481130b587db87716a3c7c4cd233a
  Branch:    recursor-refactor (uncommited files present)
  Location:  Verifier.SAW.Simulator.Prims
  Message:   atOp: expected Nat, got <extra> fresh:i#786 : PrimName {primVarIndex = 133, primName = Prelude.Nat, primType = sort 0}
CallStack (from HasCallStack):
  panic, called at src/Verifier/SAW/Utils.hs:37:9 in saw-core-0.1-inplace:Verifier.SAW.Utils
  panic, called at src/Verifier/SAW/Simulator/Prims.hs:376:13 in saw-core-0.1-inplace:Verifier.SAW.Simulator.Prims
  panic, called at src/Verifier/SAW/Simulator/Prims.hs:705:12 in saw-core-0.1-inplace:Verifier.SAW.Simulator.Prims
%< ---------------------------------------------------

Basically, the problem is that the at primitive is too strict about the form it expects from its arguments and panics when it should instead continue with term reconstruction. Some of the other sequence primitives have similar problems. We'll need to reorganize these primitives so they don't panic in these situations.