docs: function validateSentence(parsed) {
    const sentences = parsed.sentences();
    return [
      !isSingleSentence(sentences) ? 'Must be one and only one sentence.' : null,
      !isNotTooShort(parsed) ? 'Must be at least two words long.' : null,
      !isNotPunctuated(sentences) ? 'Must not have any end punctuation.' : null,
    ].filter(Boolean);
  }