function areMergeable(oldDataStructure, newDataStructure) {
    var oldSeq = Seq(oldDataStructure);
    var newSeq = Seq(newDataStructure);
    // This logic assumes that a sequence can only fall into one of the three
    // categories mentioned above (since there's no `isSetLike()` method).
    return (
      isIndexed(oldSeq) === isIndexed(newSeq) &&
      isKeyed(oldSeq) === isKeyed(newSeq)
    );
  }
It's unclear what the desired behavior is for merging two collections that
fall into separate categories between keyed, indexed, or set-like, so we only
consider them mergeable if they fall into the same category.