What are the implications of a plea of guilty in a criminal case?

"This phase of the process of criminal justice, and the adjudicative element inherent in accepting a plea of guilty, must be attended by safeguards to ensure the defendant what is reasonably due in the circumstances. Those circumstances will vary, but a constant factor is that when a plea rests in any significant degree on a promise or agreement of the prosecutor, so that it can be said to be part of the inducement or consideration, such promise must be fulfilled." (Santobello v. New York (1971) 404 U.S. 257, 262, 92 S.Ct. 495, 499, 30 L.Ed.2d 427.)