def frequency_prior_score(mention: str, candidate: dict) -> float:
    """
    Fraction of unqualified mentions of `mention` that refer to this candidate
    in the training corpus. Stored as a per-mention-string dict on the node.
    """
    priors = candidate.get("mention_priors", {})
    return priors.get(mention.lower(), candidate.get("default_frequency_prior", 0.0))