That's identifying all the nested exposures for the exposure currently being evaluated, based on the hierarchical naming convention I'm applying to the attributes (see line 140).

For nested exposures to work, we need to (a) keep track of the nesting hierarchy, and (b) ensure that we don't have conflicts with different nested exposures that happen to share an attribute name.  I figured the simplest solution would be to leave the flat structure of the `@exposures` hash alone and apply a hierarchical naming convention to the hash keys.

Consider the following nested exposures:

```ruby
expose :awesome do
  expose :nested do
    expose :moar_nested
  end
  expose :another_nested do
    expose :moar_nested
  end
end
expose :regular_thing
```

My modifications produce the following `@exposures` hash:

```ruby
{
  awesome: {},
  awesome__nested: {},
  awesome__nested__moar_nested: {},
  awesome__another_nested: {},
  awesome__another_nested__moar_nested: {},
  regular_thing: {}
}
```

To get the exposures nested under `:awesome`, we just find all hash keys whose string representation matches `/^awesome__/`.

I also added a `name_for` method (see line 383) when we just need the local name of the attribute.