If Costello is permitted to testify to conversations with Michael Cohen in 2018, Justice Merchan will be permitting another irrelevant sideshow in this trial. Michael Cohen is not on trial. This impeachment goes to nothing relevant to any element of any charge that the defendant, Donald Trump, is on trial for. You can't just impeach a witness on everything, e.g., "Have you ever masturbated?" "No." The defense calls Robert Costello to say Cohen admitted on the phone to him masturbating. The subject matter has to be relevant.