Mistake in oblig

Exercise O1.2 a) refers to ¡°Definition 28 of Section 3.2¡± for the sequent calculus.  This is a typos, and Ben-Ari contains no such definition.

What is meant is the propositional LK calculus presented in Lecture 2, slides 34 and 35.

In Ben-Ari, the calculus is defined (with a slightly different notation) in Def 3.51, with rules presented in Fig. 3.2.