unionRankCovers = (compare, covers) => {
  /**
   * @param {RankCover} a
   * @param {RankCover} b
   * @returns {RankCover}
   */
  const unionRankCoverPair = ([leftA, rightA], [leftB, rightB]) => [
    minRank(compare, leftA, leftB),
    maxRank(compare, rightA, rightB),
  ];
  return covers.reduce(unionRankCoverPair, ['{', '']);
}
@param {RankCompare} compare
@param {RankCover[]} covers
@returns {RankCover}