References:
        https://developer.github.com/v3/pulls/#get-a-single-pull-request
        https://developer.github.com/v4/enum/mergestatestatus/

def classify_pr_synced_state(pr: PullRequestDetails) -> Optional[bool]:
    """
    References:
        https://developer.github.com/v3/pulls/#get-a-single-pull-request
        https://developer.github.com/v4/enum/mergestatestatus/
    """
    state = pr.payload['mergeable_state'].lower()
    classification = {
        'behind': False,
        'clean': True,
    }
    return classification.get(state, None)