getIndexCover = (sorted, compare, [leftKey, rightKey]) => {
  assertRankSorted(sorted, compare);
  const leftIndex = rankSearch(sorted, compare, leftKey, 'leftMost');
  const rightIndex = rankSearch(sorted, compare, rightKey, 'rightMost');
  return [leftIndex, rightIndex];
}
@param {Passable[]} sorted
@param {RankCompare} compare
@param {RankCover} rankCover
@returns {IndexCover}