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

theorem Th12:
  a 'eqv' b = 'not' (a 'xor' b)
proof
  'not' (a 'xor' b) = 'not' 'not' (a 'eqv' b) by BVFUNC_6:85;
  hence thesis;
end;
