(a) A ground for relief is “previously determined” if a court of competent jurisdiction has ruled on the merits after a full hearing.
(b) A ground for relief is “waived” if the petitioner knowingly and understandingly fails to present it for determination in any proceeding before a court of competent jurisdiction in which the ground could have been presented. There is a rebuttable presumption that a ground for relief not raised in any such proceeding that was held was waived.