todo md
  <!--
- [ ] write changes engine in php
  - [ ] predict all possible unexpected situation (write here)
    - lesson starts with
      - 0
      - -1
      - -2
    - lessons on saturday / sunday 💀💀
    - missing lesson (like 1,2, 4; also don't forger about this in editor)
- [x] start develop editor ui -->
