variables p q : Prop
check ¬(p → q)
check ¬p → q
check (¬p) → q
check ¬(p → q)
check p ↔ q
check ¬(p ↔ q)
check ¬p ↔ q
check (¬p) ↔ q
check (p → q) ↔ q
check p → (q ↔ q)
