code: function checkIndexFieldsMatch(indexFields, sortOrder, fields) {
    if (sortOrder) {
      // array has to be a strict subarray of index array. furthermore,
      // the sortOrder fields need to all be represented in the index
      var sortMatches = utils.oneArrayIsStrictSubArrayOfOther(sortOrder, indexFields);
      var selectorMatches = utils.oneArrayIsSubArrayOfOther(fields, indexFields);
  
      return sortMatches && selectorMatches;
    }
  
    // all of the user's specified fields still need to be
    // on the left side of the index array, although the order
    // doesn't matter
    return utils.oneSetIsSubArrayOfOther(fields, indexFields);
  }
docs: check that at least one field in the user's query is represented in the index. order matters in the case of sorts