The witnesses do not become zomemim unless they discredit them personally. How so? [If] they said, “We testify against so-and-so that he killed someone”; [and others] said to them, “How can you testify [so], when this murder victim,” or “this murderer,” “was with us on that day in such and such a place” —- these are not [considered] zomemim. But [if] they said to them, “How can you testify [so], when you were with us on that day in such and such a place,” these are [considered] zomemim, and they are executed by their testimony.