Exercise 3.15.  Draw box-and-pointer diagrams to explain the effect of 
set-to-wow! on the structures z1 and z2 above.

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As in figures 3.16 and 3.17, except that in z2, one of the two pointers 
to a becomes a pointer to wow.  In z1 there is only one pointer to a, 
which is changed to point to wow.