Abel (D) was on trial for robbery. D and two cohorts were indicted for bank robbery. The other parties pleaded guilty, but D went to trial. Ehle agreed to testify against D. D informed the District Court that he would seek impeach Ehle's testimony with that of one Mills, who would testify that after the robbery Ehle had admitted to Mills that Ehle intended to implicate D falsely, in order to receive favorable treatment from the Government. The prosecutor then disclosed that he intended to discredit Mills' testimony by calling Ehle back to the stand to testify that respondent, Mills, and Ehle were all members of a secret prison gang that was sworn to perjury and self-protection on each member's behalf. Mills denied knowledge of the prison gang, and the Court allowed P to recall Ehle, who testified that he, D, and Mills were members of the prison gang and described the gang and its tenets. The jury convicted D. The Court of Appeals reversed, holding that Ehle's rebuttal testimony was admitted not just to show that respondent's and Mills' membership in the prison gang might cause Mills to color his testimony but also to show that because Mills belonged to the gang, he must be lying on the stand. The court further held that Ehle's testimony implicated respondent as a member of the gang, but that since respondent did not take the stand, the testimony could not have been offered to impeach him and prejudiced him 'by mere association.' The Government (P) appealed.