assertion vs. presupposition

a. John is absent again today.
b. Today is not the first time that John is absent.
c. John is absent today, and that has happened before.

what is presupposed in all theses?
a. John was absent before.
b. John is absent today.
c. none.

what is asserted (new info)?
a. John is absent today.
b. John is absent before.
c. John is absent today, and that John was absent before.

quite interesting, isn't it?