Function optimal_answer_set 

pub fn optimal_answer_set(
    atoms: &[u64],
    rules: &[AspRule],
    weak: &[WeakConstraint],
    buf: &mut [u64],
) -> Option<(u64, i64)>

The optimal answer set: the stable model minimising total weak-constraint penalty. Returns (model_bitmask, penalty), or None if the program has no stable model. buf is scratch for the enumerated answer sets.