KEEP star-star-star

In #211, we added a way to tell the compiler that even though a paragraph was *edited*, the regulation authors only intended its *children* to be modified.

The solution there (using a "children" field) worked okay, but didn't solve the more general problem. There might be "* * *" paragraphs litered throughout a change, indicating that the associated paragraph should remain, but its children (or neighbors) might be modified.

This patch implements that via a 'KEEP' verb, which alerts the compiler that the associated nodes should not change (even when their parent is replaced).

This technique is also helpful when a change only modifies *some* of a paragraph's children, as it retains the original set. For example, if the original tree contained 12-b, 12-b-1, 12-b-1-i through ii, an 12-b-2:

```
Revising 12(b)

(b) Content
(1) * * *
* * * * *
(ii) Content
(2) Content
```
Would result in the same nodes, but modified text for 12-b, 12-b-1-ii, 12-b-2