Due to a copy and paste error, tree (d) on the key for homework 7 had MP above NegP, but NegP is supposed to be above MP. I have posted yet another fixed version of the key, but the grader also graded it based on the erroneous key, so it is quite likely that your homework has an extra point taken off for that when it shouldn’t have been. Let me know if you haven’t already if this happened to you, I’ll adjust the score.