* implement LiteralStringVar and format/retrieval mechanism
* use create safe
* add cached properties to ConcatVarOperation
* fix caches
* also include self
* fix inconsistencies in typings
* use default factory not default
* add missing docstring
* experiment with immutable var data
* solve pydantic issues
* add sorted function
* missing docs
* forgot ellipses
* give up on frozen
* dang it darglint
* fix string serialization bugs and remove unused code
* add returns statement
* whitespace moment
* add simple test for string concat
* export ConcatVarOperation