theorem Th47:
  Ex(x,'not' 'not' p) => Ex(x,p) is valid & Ex(x,p) => Ex(x,'not'
  'not' p) is valid
proof
  All(x,'not' 'not' p => p) is valid & All(x,p => 'not' 'not' p) is valid
  by Th23;
  hence thesis by Th35;
end;
