In State v. Vargas, 127 Ariz. 59, 61, 618 P.2d 229, 231 (1980) the Arizona Supreme Court held that Rule 410 prohibited the state from introducing, either in its case-in-chief or for impeachment purposes, a letter the defendant had signed "during plea negotiations," that is, "during discussions concerning a possible plea of guilty." The court observed that "permitting the use of plea discussions for impeachment would have a strong chilling effect on plea negotiations." Vargas, 127 Ariz. at 61, 618 P.2d at 231.