Function evaluate 

pub fn evaluate(subject: &Subject<'_>, tree: &PolicyTree) -> Verdict

Evaluate a subject against a PolicyTree, in the tree’s presentational order.

Any EvalError from a candidate’s evaluator, and any candidate whose predicate is Predicate::Unusable — see PolicyTree::decide — is treated as a refusal: fail-closed, and scoped to exactly the subject that predicate was asked about, never escalated to Outcome::Freeze on its own. This is deny-only monotonicity’s third consequence from plan/policy.md: a predicate that cannot answer can only make the verdict more restrictive, never less.

The decision channel short-circuits: once a Outcome::Freeze is seen, no severity can outrank it, so remaining candidates are not asked to decide. (They may still be reached by PolicyTree::observation_candidates — a separate walk this function does not perform; see that method’s doc comment for why observation is independent of decision.)