import standard
using tactic

theorem tst (a b : Prop) (H : ¬ a ∨ ¬ b) (Hb : b) : ¬ a ∧ b
:= by apply and_intro;
      apply not_intro;
      exact
        (assume Ha, or_elim H
          (assume Hna, absurd Ha Hna)
          (assume Hnb, absurd Hb Hnb));
      assumption