What law was abel fields convicted under?

Abel (defendant) was charged with a bank robbery. One of his cohorts in the robbery, Ehle, pled guilty to the robbery and agreed to testify against Abel. At trial, Abel sought to rebut Ehle’s testimony by introducing the testimony of Mills who stated that after the robbery, Ehle admitted to Mills that Ehle intended to testify falsely against Abel to get a reduced sentence. In response to this, the prosecution sought to call Ehle back to the stand to testify that he, Able, and Mills were all part of a secret prison gang that required its members to deny its existence and commit perjury for each other. Abel objected to Ehle’s testimony about the gang. The district court admitted the testimony and convicted Abel. The United States Court of Appeals for the Ninth Circuit reversed. The United States Supreme Court granted certiorari.