Function holds_by_default 

pub fn holds_by_default(facts: &[NQuin], goal: &NQuin) -> bool

Negation-as-failure / closed-world assumption: a proposition holds “by default” (its negation is concluded) exactly when it CANNOT be proven from the closed set of facts — i.e. the (subject, predicate, object) triple is absent. This is the non-monotonic primitive the positive forward-chainer (fire_guard_rules) cannot express; the agent-honesty guard’s “Unverified until proven” is an instance. Zero-heap (single linear scan).