For example, in
```
void M(object x)
{
var y = x == null ? 1 : 2;
if (y == 2)
x.ToString();
}
```
the guard `y == 2` implies that the guard `x == null` must be false,
as the assignment of `2` to `y` is unique.
For example, in
```
void M(object x)
{
var y = x != null ? "" : null;
if (y != null)
x.ToString();
}
```
the guard `y != null` implies that the guard `x != null` must be true.
