No apparent support for Fortran "external" attribute

I believe this is another way to specify a dependency which does not seem to trip the parser and generate the dependency.

Start by locating the Fortran parser and the code that generates dependencies, then trace how the external attribute is handled. Reproduce the issue with a Fortran example using external and confirm that the expected dependency is generated when the work is complete.

Written by the indexing model from the issue text.