theorem Th5:
  (p => q) => ('not'(q '&' r) => 'not'(p '&' r)) in Cn(X)
proof
 T is being_a_theory & X c= T implies
  (p => q) => ('not'(q '&' r) => 'not'(p '&' r)) in T;
  hence thesis by Def2;
end;
