Now generating dynamic GEXF files, but they're not quite working in Gephi
Got dynamic graphs working (but need to rewrite).

Got the dynamic graph generation working, readable by Gephi, etc.
However, it turns out what I didn't doesn't do what I want.
Gephi requires that each node have a different id, even if they
are at non-overlapping times.  Likewise for edges.  That's what I
implemented.  But that means that Gephi is unaware of the continuity
between different "nodes".

So what I need to do, to have dynamic popco graphs, is something very
different:

(a) nodes and edges have a start time when they first exist, with
no end time (they exist indefinitely).

(b) GEXF weights change over time (can I do that?) and other attributes
such as activations, popco-weights, colors, and sizes change over time--
for the same node.  i.e. there is a node that never goes away, but
its attributes are time-indexed, which is one feature that GEXF allows.
That's what will do what I want.

I also wonder whether I should be using atoms for id numbers and all
that.  Bit of a kludge (from a functional point of view), but it made
things much simpler.  Though I suppose it's innocuous in this context.