Should \source be replaced with "type-specific" directives?

    submodule stuff
    class Thing
    record Stuff

If there's a distinction between classes and records,
should they then have different names?

* Additional file extension? E.g. "Thing.class.slul"
  But this could get incorrectly matches with Java .class files.
* Case distinction, e.g. SomeClass.slul vs some_record.slul
* Prefix letter like in Pascal, e.g. CSomeClass.slul vs RSomeRecord.slul
  But this will make letters "C" and "R" overfilled when sorting.
* Suffix letter: SomeC.slul SomeR.slul


Can this be done in a good way with "index files" instead of module headers?
Yes, if either of thse:
* the source files have different names.
* there are separate index files
* the index files contain the kind of source file
  (record type, class/object type or multiple types)

Filename-based types and directories:
* The directory part could be skipped (best solution?)
* Or the directory could be prepended as some kind of namespace.
* Or nested directories could be forbidden (except for submodule usage)

Should modules be able to access each other?
* They will probably need to in some cases.
* Depending on the top-level module is probably a bad idea and should not be allowed.
* Should there be internal module dependencies?
    - Perhaps a `intra_imports.index` file?
             or `cross_imports.index`
             or `submodule_imports.index`
             or `used_submodules.index` (best?)
    - Should the top-level module implicitly be able to use every module?