docs: function isFileGroup(files) {
    const basePatternString = `~${files.length}/nth/`;
    const mapExpression = (val, idx) => new RegExp(`${basePatternString}${idx}/$`);
    const expressions = Array.from({ length: files.length }, mapExpression);
    return expressions.every(exp => files.some(url => exp.test(url)));
  }