Code coverage for lfd and lff too low

[lhstrh]

Currently, code coverage for lfd and lff is 39% and 60%, respectively. This needs to be improved.

[ghost]

@lhstrh Can you assign this to me ?

[lhstrh]

Let me know if you need any help, @areyeswhy. We can chat on Zulip if you like.

[cmnrd]

I am not sure if there is much we can do about the lfd code coverage. We would somehow need to inject errors into the diagram synthesis to see if the error reporting works.