propagation clustering tool.  For 456 cities in the US, we have the mean
% travel time between airports in those cities, along with their latitude
% and longitude.
load_gaimc_graph('airports')

%%
% For some reason, the data is stored with the negative travel time between
% cities.  (I believe this is so that closer cities have larger edges
% between them.)  But for a minimum spanning tree, we want the actual
% travel time between cities.
A = -A;

%%
% Now, we just call MST and look at the result.
% T = mst