Function flatten 

pub fn flatten<A>(
    framework: &BipolarFramework<A>,
) -> Result<ArgumentationFramework<A>, Error>
where
    A: Clone + Eq + Hash + Debug,

Build a argumentation::ArgumentationFramework from a BipolarFramework whose attack relation is the closed attack set.

Propagates argumentation::Error from add_attack calls, but in practice this only fires if the argument universe is inconsistent (an edge references an argument that wasn’t registered), which cannot happen here because closed_attacks only produces edges between arguments already in the framework.