pub fn difference(&self, other: &Self) -> Self {
        assert!(
            ::std::ptr::eq(self.graph.0, other.graph.0),
            "package graphs passed into difference() match"
        );
        Self {
            graph: self.graph,
            core: self.core.difference(&other.core),
        }
    }
Returns a `PackageSet` that contains all packages present in `self` but not `other`.

## Panics

Panics if the package graphs associated with `self` and `other` don't match.
guppy/src/graph/resolve.rs