reserve Y for non empty set;

theorem
  for a,b being Function of Y,BOOLEAN holds a 'eqv' b=I_el(Y)
  implies 'not' a 'eqv' 'not' b=I_el(Y)
proof
  let a,b be Function of Y,BOOLEAN;
  assume (a 'eqv' b)=I_el(Y);
  then a = b by BVFUNC_1:17;
  hence thesis by BVFUNC_1:17;
end;
