Matthew,
Add two more tests:
-gltLabel 4 grp_task_1vs2 -gltCode 4 "group : -1* healthy 1* diseased strategy : 1*task_1 -1*task_2 " \
-gltLabel 5 grp_task_2vs3 -gltCode 5 "group : -1* healthy 1* diseased strategy : 1*task_2 -1*task_3 " \
Then the conjunction of the above two tests in the positive (or negative) domain would be what you're looking for.
Gang
