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.

Not bug deals but adjustments of details.

* chain/src/item/default/splone.rs: Previously when we split nodes, we
  always clone the parent if the labels differ.  This turns out to be
  incorrect if the new label is open whereas the old label is closed.
  In that case, the old parent should not contain the new node as a
  child, as a closed node should not contain an open node.

  I am not yet entirely sure this fix is correct, so more test await
  us.

Now the binding part is finished.

What remains is a bug encountered when planting a fragment to the
forest which intersects a packed node, which would lead to invalid
forests.  This will also cause problem when planting a packed
fragment, but until now my testing grammars do not produce packed
fragments, so this problem is not encountered yet.

I am still figuring out efficient ways to solve this problem.