Ack -- problem #23 (the fill-in-the-blanks for 
   x-has-1  |-  (w-unsafe v y-unsafe)
) isn't correct as written.  We'll scratch that problem
entirely from the assignment.




As extra credit, we'll add two possible options:

(option 23a) -- 2pts -- complete the proof, 
but re-write step 5 entirely.  (It can be similar to step 4.)

(option 23b) -- 3pts -- you can complete the problem by
adding line 5-and-a-half:
    5-and-half: _____________  [by (phi v -phi), as shown in class thurs.]
The remaining blanks are filled as indicated.


(You can get extra-credit for one option or the other, but not both.)