Crash when finding size of graph

```
Disco> |emptyGraph|
disco: Impossible! numOp1' on non-VNum VGraph Empty
CallStack (from HasCallStack):
  error, called at src/Disco/Interpret/CESK.hs:507:15 in disco-0.1.6-inplace:Disco.Interpret.CESK
```

Not sure if this is supposed to be a type error, or if it's supposed to work.