most importantly—
• if this problem still confuses you after a while, just walk away from it. then come back to it after a couple of weeks.
• at that point you'll have 'new eyes', and you might find that you suddenly have a better intuition for what's going on here.
• if the problem still confuses you at that point, then just...
...smile,
and
...ignore it.
this single problem is, quite clearly, of little importance overall. (i think it's the only GMAC problem ever in this particular format.)
do not let it distract you from more fundamental concerns.