Given this simple implementation of string calculator, mid way, how would you refactor this, based on SOLID principles? and how far would you go?

Please note what principles you think this code breaks, and why it is important that it implement them, if at all. show how you refactored it in a gist.

Post your own gists and tweet them under hashtag #solidkata and we can start a discussion on this. later I might use them in my videos, courses and presentations.