Check Hendrik Tews' ProofTree visualization code

**[Original report](https://bitbucket.org/Carst/proviola-source/issue/12) by Carst Tankink (Bitbucket: [Carst](https://bitbucket.org/Carst), GitHub: [Carst](https://github.com/Carst)).**

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Might help in generating subgoal-frames.
