bbi nonmem covcor call on top-level model will fail with message like "no file present /path/to/mod/mod.cov".
The submodels each have a .cov file. Is there something reasonable we could return there? bbi nonmem covcor call for each submodel, returning a list? (It's unfortunate that the return value type would be different than that of plain NONMEM models.)
Initial plan is for is to add a tailored method that gives a more appropriate error.
[ follow-up to gh-36 and gh-126 ]
https://github.com/metrumresearchgroup/bbr.bayes/issues/36#issuecomment-1939304096: