I am wondering what properties are attached to the various levels of a
hierarchy and how to retrieve it.
As an example, I would like to know how to get the one-character-code
that is attached to a chain.
There should never be a need to mess with keys and attributes in the way
you're proposing - the decorators should always provide suitable methods.
I have the feeling, this is not strictly feasible, though it is possible
to get it as the last character in the particle name of the chain :
The particle name is a tag for the benefit of you, the human. There is
no guarantee that it ends in the chain ID, since you can set it to
anything you want.