Once this first pass is done, we can examine the set of candidates. If
it is a singleton set, then we are done: this is the only impl in
scope that could possibly apply. Otherwise, we can **winnow** down the set
of candidates by using where clauses and other conditions. Winnowing uses
`evaluate_candidate` to check whether the nested obligations may apply. 
If this still leaves more than 1 candidate, we use ` fn candidate_should_be_dropped_in_favor_of` 
to prefer some candidates over others. 


If this reduced set yields a single, unambiguous entry, we're good to go,
otherwise the result is considered ambiguous.

#### Winnowing: Resolving ambiguities

But what happens if there are multiple impls where all the types
unify? Consider this example:

```rust,ignore
trait Get {
    fn get(&self) -> Self;
}

impl<T: Copy> Get for T {
    fn get(&self) -> T {
        *self
    }
}

impl<T: Get> Get for Box<T> {
    fn get(&self) -> Box<T> {
        Box::new(<T>::get(self))
    }
}
```

What happens when we invoke `get(&Box::new(1_u16))`, for example? In this
case, the `Self` type is `Box<u16>` – that unifies with both impls,
because the first applies to all types `T`, and the second to all
`Box<T>`. In order for this to be unambiguous, the compiler does a *winnowing*
pass that considers `where` clauses
and attempts to remove candidates. In this case, the first impl only
applies if `Box<u16> : Copy`, which doesn't hold. After winnowing,
then, we are left with just one candidate, so we can proceed.

#### `where` clauses

Besides an impl, the other major way to resolve an obligation is via a
where clause. The selection process is always given a [parameter
environment] which contains a list of where clauses, which are
basically obligations that we can assume are satisfiable. We will iterate
over that list and check whether our current obligation can be found
in that list. If so, it is considered satisfied. More precisely, we
want to check whether there is a where-clause obligation that is for
the same trait (or some subtrait) and which can match against the obligation.


Consider this simple example:

```rust,ignore
trait A1 {
    fn do_a1(&self);
}
trait A2 : A1 { ... }

trait B {
    fn do_b(&self);
}

fn foo<X:A2+B>(x: X) {
    x.do_a1(); // (*)
    x.do_b();  // (#)