DRAFT: rewrite of 4.1.4.1 Activity as the object

Discussed with Russel about this and decided it's best to make a PR,
but...
- this one needs thorough checking, it's very complex.
- the second table is not entirely complete (third column Opt/Req) and
should be checked meticulously

Made the PR because of deadline, but it's really not ready until some
other expert improves this.