reserve Y for non empty set;

theorem Th26:
  for a,b being Function of Y,BOOLEAN holds a 'imp' (a 'or' b)= I_el(Y)
proof
  let a,b be Function of Y,BOOLEAN;
  for x being Element of Y holds (a 'imp' (a 'or' b)).x=TRUE
  proof
    let x be Element of Y;
    (a 'imp' (a 'or' b)).x ='not' a.x 'or' (a 'or' b).x by BVFUNC_1:def 8
      .='not' a.x 'or' (a.x 'or' b.x) by BVFUNC_1:def 4
      .=('not' a.x 'or' a.x) 'or' b.x
      .=TRUE 'or' b.x by XBOOLEAN:102
      .=TRUE;
    hence thesis;
  end;
  hence thesis by BVFUNC_1:def 11;
end;
