This is the first time I post anything on the forum. I started with Tomassi's Logic and unfortunately I have been unable to solve some of its problems. One I get immediately stuck with is this one:
$$\lnot(p\to q)\vdash p \land\lnot q$$
I have to solve it by natural deduction and the only rules I know are: assumptions, modus ponendo ponens, modus tollendo tollens, double negation, reductio ad absurdum, conditional proof, v-introduction, v-elimination, introduction, and elimination. Tomassi's proof consists of 12 steps.
Moreover, I don't see how to proceed because of the negations on the outside of the parentheses.
Thanks for the help!