Function is_optional 

pub fn is_optional(norms: &[NQuin], party: u64, action: u64) -> bool

True iff action φ is optional / indifferent for party: ¬O(φ) ∧ ¬F(φ) — no active (non-defeater) obligation and no prohibition over (party, action) in the norm slice. (An explicit OP_OPTIONAL assertion also counts.)