Function bipolar_preferred_extensions 

pub fn bipolar_preferred_extensions<A>(
    framework: &BipolarFramework<A>,
) -> Result<Vec<HashSet<A>>, Error>
where
    A: Clone + Eq + Hash + Debug + Ord,

All bipolar preferred extensions under necessary-support semantics.

Pipeline: flatten → Dung preferred extensions → support-closure filter. The filter may drop candidates that are Dung-preferred but not support-closed. It does NOT promote smaller subsets in their place; if every Dung-preferred extension is filtered out, the result is empty.