TODOs:

- Once this is resolved, report back to Emil, e.g. in the Fast Downward mailing
list thread, replying to Malte's email that referenced this issue (21.05.2019
16:54 CEST).

I think the warnings would definitely be useful, though. Perhaps we can make
them an optional feature.

Making this part of the grounding algorithm would have the advantage that we
could also eliminate other, then unreachable action.

My first impulse was to integrate the testing of well-definedness in the
grounding algorithm (for efficiency), but then we cannot easily get such
warnings because the failing actions are never generated.

This was mentioned by Emil Keyder on the mailing list.

If some action refers to an uninitialized numeric expression, the translator
stops with "AssertionError: Could not find instantiation for PNE!". The correct
behaviour would be to simply not instantiate this action.

Maybe we should in this case include an output that the action was removed,
because otherwise this behaviour could easily hide modelling mistakes.