Wow, that's tough!!!
Alot depends on how you define "sequel"...
To me, "Part 2" of a defined story line isn't exactly a "sequel" and breaking one big long story into multiple parts doesn't quite qualify as "sequels".
But the online dictionary seems to disagree:
1. Something that follows; a continuation.
2. A literary, dramatic, or cinematic work whose narrative continues that of a preexisting work.
3. A result or consequence.