Now I tested and discussed with claude, and had a working version of
this project.  The rest is to port the claude version to my own
writings.  So this serves as the starting point before that attempt.

In the new attempt the design is better, and a number of bugs were
found and supposedly fixed, so let us see how this goes at the end.

I think the main functionalities are already there, but the lack of
querying capabilities makes this not very useful at the moment.

This commit is right before an attempt to add this missing piece.

It seems to be complete now, but still awaits more tests to see where
the errors are, which should be plenty, haha.

I seem to have finished the implementation of forests.  Now it remains
the implementation of the chain-rule machine, of which I have a rough
plan now.

I am about to re-start my system, so I save before any crashes
happen.

Now I have a new type of labelled graphs, which can index vertices by
labels, but not index edges by labels.  The biggest difference is that
I do not have to keep a hashmap of edge targets by labels, and I do
not have to guard against the duplication of nodes with the same set
of edges.  I guard against nodes with the same label, though.

Also, in this graph, both vertices and edges have one label at a time,
whereas in the previous labelled graph there can be a multitude of
edges between the same source and target nodes, but with different
labels.

Now it remains to test this type of graphs, and to think through how
we attach forest fragments to nondeterministic finite automata edges,
and how to join forest fragments together while skipping nullable
edges, in order to finish the "compilation" part.