Function possible 

pub fn possible(
    graph: &[NQuin],
    world: u64,
    prop: u64,
    accesses: u64,
    holds: u64,
) -> bool

General modal logic over a Kripke frame — the shared modal substrate (□/◇) that the specific modalities (deontic, epistemic) specialise. Accessibility is (world →accesses→ world') (predicate == accesses); a world satisfies a proposition when (world, holds, prop) is present. Zero-heap (linear scans).

With S4 (reflexive+transitive) or S5 (equivalence) accessibility supplied as edges, this yields the corresponding modal system; the evaluator itself is frame-agnostic (it reads whatever accessibility edges are asserted). ◇φ (possible) — SOME world accessible from world satisfies prop.