emptyFallback = (result, alternative, errorMessage) => {
    if (result !== EMPTY_SYGIL) {
        return result;
    }
    if (alternative === THROW_IF_EMPTY) {
        throw new Error(errorMessage);
    }
    return alternative;
}
A private sygil/sentinel value that shouldn't ever be present in a sequence, and so can be used as a placeholder for
"not set yet" (unlike undefined or null, which are allowed to appear in sequences).