code: function compoundCompareThings (fields) {
    return function (a, b) {
      var i, len, comparison;
  
      // undefined
      if (a === undefined) { return b === undefined ? 0 : -1; }
      if (b === undefined) { return a === undefined ? 0 : 1; }
  
      // null
      if (a === null) { return b === null ? 0 : -1; }
      if (b === null) { return a === null ? 0 : 1; }
  
      for (i = 0, len = fields.length; i < len; i++) {
        comparison = compareThings(a[fields[i]], b[fields[i]]);
        if (comparison !== 0) { return comparison; }
      }
  
      return 0;
    };
  }
docs: Used primarily in compound indexes. Returns a comparison function usable as
  an Index's compareKeys function.