`--where` determines which packages are used to find the transitive closure. By default, the packages which satisfy the `--where` preds are placed into the package indices but only those packages *not* their dependencies (which get generated but are hidden unless you browse to them through another package).

`--index` lets you select some subset of `--where`'s transitive closure and then generates indices which include those packages.

This is useful when you want to generate a website that includes the transitive closure of a few packages with the `pkg:*` pred but then want to generate index files which contain more than just the packages that you matched. For instance,

```
opam2web -c content/ -o www --where pkg:core --where pkg:core_bench --where pkg:core_extended --where pkg:async --where pkg:utop --index all
```

would generate a web site with all of the RWO packages in its index given only those 5 top level packages. It's sorta like a sub-SELECT?

Perhaps I need to write better docs...