URBANDALE — It was arguably his finest and best-received moment of the night. It came at the end of the event.And Ben Carson said it was all off the cuff.Carson, a retired neurosurgeon, joked during his closing statement at Thursday night’s Republican presidential candidate debate in Cleveland that what separates him from the rest of the crowded GOP field is he has separated Siamese twins, operated on children still in the womb and removed half a person’s brain.The next day in Iowa, Carson said the jovial moment was unplanned.“That was not planned.