reserve Y for non empty set,
  a,b,c,d for Function of Y,BOOLEAN;

theorem Th15:
  (a 'or' b) '&' ('not' a 'or' 'not' b) = ('not' a '&' b) 'or' (a '&' 'not' b)
proof
  a 'xor' b = ('not' a '&' b) 'or' (a '&' 'not' b) by BVFUNC_4:9;
  hence thesis by BVFUNC_6:86;
end;
