to see by using a kind of generic "logic" example:

```text
exists<X> {
   forall<Y> { ... /* Y is in U1 ... */ }
   forall<Z> { ... /* Z is in U2 ... */ }
}
```

Here, the only way for the two foralls to interact would be through X,
but neither Y nor Z are in scope when X is declared, so its value
cannot reference either of them.

## Universes and placeholder region elements

But where does that error come from?  The way it happens is like this.
When we are constructing the region inference context, we can tell
from the type inference context how many placeholder variables exist
(the `InferCtxt` has an internal counter). For each of those, we
create a corresponding universal region variable `!n` and a "region
element" `placeholder(n)`. This corresponds to "some unknown set of other
elements". The value of `!n` is `{placeholder(n)}`.

At the same time, we also give each existential variable a
**universe** (also taken from the `InferCtxt`). This universe
determines which placeholder elements may appear in its value: For
example, a variable in universe U3 may name `placeholder(1)`, `placeholder(2)`, and
`placeholder(3)`, but not `placeholder(4)`. Note that the universe of an inference
variable controls what region elements **can** appear in its value; it
does not say region elements **will** appear.

## Placeholders and outlives constraints

In the region inference engine, outlives constraints have the form:

```text
V1: V2 @ P
```

where `V1` and `V2` are region indices, and hence map to some region
variable (which may be universally or existentially quantified). The
`P` here is a "point" in the control-flow graph; it's not important
for this section. This variable will have a universe, so let's call
those universes `U(V1)` and `U(V2)` respectively. (Actually, the only
one we are going to care about is `U(V1)`.)

When we encounter this constraint, the ordinary procedure is to start
a DFS from `P`. We keep walking so long as the nodes we are walking
are present in `value(V2)` and we add those nodes to `value(V1)`. If
we reach a return point, we add in any `end(X)` elements. That part
remains unchanged.

But then *after that* we want to iterate over the placeholder `placeholder(x)`
elements in V2 (each of those must be visible to `U(V2)`, but we
should be able to just assume that is true, we don't have to check
it). We have to ensure that `value(V1)` outlives each of those
placeholder elements.

Now there are two ways that could happen. First, if `U(V1)` can see
the universe `x` (i.e., `x <= U(V1)`), then we can just add `placeholder(x)`
to `value(V1)` and be done. But if not, then we have to approximate:
we may not know what set of elements `placeholder(x)` represents, but we
should be able to compute some sort of **upper bound** B for it –
some region B that outlives `placeholder(x)`. For now, we'll just use
`'static` for that (since it outlives everything) – in the future, we
can sometimes be smarter here (and in fact we have code for doing this
already in other contexts). Moreover, since `'static` is in the root
universe U0, we know that all variables can see it – so basically if
we find that `value(V2)` contains `placeholder(x)` for some universe `x`
that `V1` can't see, then we force `V1` to `'static`.

## Extending the "universal regions" check

After all constraints have been propagated, the NLL region inference
has one final check, where it goes over the values that wound up being
computed for each universal region and checks that they did not get
'too large'. In our case, we will go through each placeholder region
and check that it contains *only* the `placeholder(u)` element it is known to
outlive. (Later, we might be able to know that there are relationships
between two placeholder regions and take those into account, as we do
for universal regions from the fn signature.)

Put another way, the "universal regions" check can be considered to be
checking constraints like:

```text
{placeholder(1)}: V1
```

where `{placeholder(1)}` is like a constant set, and V1 is the variable we