code: function calculateCartesianProduct(keyRangeSets) {
    goog.asserts.assert(
        keyRangeSets.length > 1,
        'Should only be called for cross-column indices.');
  
    var keyRangeSetsAsArrays = keyRangeSets.map(
        function(keyRangeSet) {
          return keyRangeSet.getValues();
        });
  
    var it = goog.iter.product.apply(null, keyRangeSetsAsArrays);
    var combinations = [];
    goog.iter.forEach(it, function(value) {
      combinations.push(value);
    });
    return combinations;
  }
docs: Finds the cartesian product of a collection of SingleKeyRangeSets.
  @param {!Array<!lf.index.SingleKeyRangeSet>} keyRangeSets A SingleKeyRangeSet
  at position i in the input array corresponds to all possible values for
  the ith dimension in the N-dimensional space (where N is the number of
  columns in the cross-column index).
  @return {!Array<!lf.index.KeyRange>} The cross-column key range combinations.