Today I'll give you the hw4 handout.  It has a written part, but you
may not have to write these unless I get some grading back to you
Monday.  There is also a program part which I'll introduce on Monday.

Today we'll look at the implementations of Kruskal's algorithm and
Prim's algorithm.  In particular there are two versions of Prim's
algorithm, depending on how we keep track of the "frontier" edges.
The more efficient version will require a new version of the MinPQ,
which lets us "reach in" and decrease an existing key.

Same files as before, also:
   ../book/IndexMinPQ.java