More intuitive treatment of `type_*` and `cat_*` mappings

Currently, mappings between sets and the category-mapping sets have to be added explicitly. This should be made "more automatic", so that a bound on a set element (e.g., an emission) can be added without the need to define the category mapping (to itself) explicitly.

No file or test is named in the issue. Start by locating the code that defines type_* and cat_* mappings, then establish a test showing a set-element bound works without an explicit self-mapping; done means that behavior is automatic.

Written by the indexing model from the issue text.