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

theorem Th14:
  a 'xor' 'not' a = I_el(Y)
proof
  thus a 'xor' 'not' a =('not' a '&' 'not' a) 'or' (a '&' 'not' 'not' a) by
BVFUNC_4:9
    .=I_el(Y) by BVFUNC_4:6;
end;
