In ROLLO - Rank-Ouroboros, LAKE & LOCKER, Ideal codes are defined in Definition 2.1.4. Thanks to the Lemma 1, it is proved that every block of ideal matrix is non-singular.
Now, why do they say "hence C can be represented under systematic form"? And in the page 11, I read "indeed, the parity-check matrix under systematic form of C if of the form..." Why has H this form?