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.

My plan is to build a little interpreter for an esoteric programming
language as the most basic application of this package, before
embarking on some more ambitious applications.

Working my way towards a user-friendly introduction to the package.

The purpose of this change is to share this trait with other crates,
such as the forth-coming "semiring" crate that will be responsible for
handling some simple semiring operations as well as the querying, in
my plans.

Previously some incorrect forest nodes will be used for planting new
nodes.  I cannot fix the root cause of their presence in the
chain-rule machine.  But I can ignore them when they are encountered.

Of course I would like to really prevent them from existing, but still
cannot figure out how.

* Cargo.toml
* chain/Cargo.toml
* grammar/Cargo.toml: Bump version.

* ChangeLog: Add a short description of the bug fix introduced in this
  version.

This crate is responsible for the users to conveniently define their
own tokenizers.  Right now the user has to tokenize the input string
manually, which is not quite what is expected from a parser, from the
view point of users.

Not bug deals but adjustments of details.