Graph Theory Problem

[Dave Smith]

I came across a graph theory problem at work today, and I decided to 
write it up to sort out the details. In the mean time, I challenge you 
to solve it:

   http://thesmithfam.org/blog/2008/12/12/graph-theory-problem/

For those who can't be troubled to follow links, here's the problem 
summarized: Given any directed graph as a set of edges, find the parent 
most node or nodes (e.g., the nodes that have the most ancestors).

--Dave