Changes

Summary

1. tuned proof;
2. merged
3. slightly more robust re-initialization;
4. isabelle_scala_script is usually found by PATH;
5. within the Isabelle environment, main executables are always within PATH;
6. avoid global state change;
7. more robust treatment of shell functions: dynamic_env recreates lost definitions on demand, e.g. after going through aggressive versions of /bin/sh -> dash;