Document Add construct

[thoni56]

In a mail conversation in [alan-if] Tristano asked about how Add works and how you can use that wrt. inheritance. Here are some highlights

• the manual indicates that you can Add , you probably cannot and possibly also should not
• there is no explanation was Add actually does, it should at least say something like "adding like as if you had written that in the class declaration directly, and adds in the same way as inheriting does"
• possibly give an example that explains that the clauses in the Add construct is just as if you had written them on the class
• describe that you can test for the Container property using "IsA"

[tajmone]

I've removed the [inheritance] part from the BNF rule (commit fb899b9 in beta7_prep branch), which was the source of confusion mentioned in the Yahoo discussion — I hope I did the right thing by editing the BNF, based on what you said in the Yahoo thread:

The gist of the situation is then that it is in the documentation and in the grammar, but not implemented. Best route forward is probably to get the docs in par with reality.

See my Issue #46 for the details.