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.

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.

Fix the broken Makefiles.

Try to fix some minor issues.

Add an intentionally ambiguous grammar for testing purposes.

It seems to work fine.

This bumping of version is insignificant.  I just find it notable that
I seem to finally obtain a version without trivial bugs.  Hooray!

This is not of much use right now, but can be helpful later.

In the process of splitting, cloning, and planting the forest, I
forgot to check whether some cloned node of the node inquestion
satisfy the condition.  This used to cause forests that violate some
fundamental assumptions.  Now this is supposed to be fixed, but more
tests await us.