%%% Assignment 2
%%% Out: Monday Sep 17
%%% Due: Monday Sep 24

1. Use truth tables to decide whether the following arguments are classically valid:

(a) 	Bush will resign or America will go to war if there is a catastrophe.
	America is going to war with Bush as president.
	There was a catastrophe.

(b) 	Bush will resign or America will go to war if there is a catastrophe.
	If there is a catastrophe while Bush is president, then America will go to war.

2. Use kripke models to prove the following:
	(a)  ||-  ~~(A | ~A)
	(b) ~~A => B ||-  A => ~~B

3. Show that the following sequent is not derivable in constructive logic:
	A => B |- ~A | B