It is an affirmative defense in a trial by court–martial that, at the time of the commission of the acts constituting the offense, the accused, as a result of a severe mental disease or defect, was unable to appreciate the nature and quality or the wrongfulness of the acts. Mental disease or defect does not otherwise constitute a defense. Procedures for the defense of lack of mental responsibility will be governed by the rules of use of courts–martial to the extent they do not conflict with State substantive law.