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

theorem
  ((a 'imp' b) '&' a) 'imp' b = I_el(Y)
proof
  ((a 'imp' b) '&' a) 'imp' b = (a '&' b) 'imp' b by BVFUNC_6:56;
  hence thesis by BVFUNC_6:40;
end;
