In the interactive mode (random=FALSE), left click to define or remove
cells, then click on red square in the bottom left corner to start
cycles. Click positions are rounded so they are not always precise.
To stop cycles, use Ctrl-C (Linux, macOS) or Esc (Windows) in the main R
window.
The code was inspired by the Github gist (which is not available anymore)
attributed to Vadim Vinichenko.