Building with mini, Part 4/9: The main loop — next plan do done

pycalc has a vision (Part 2) and a backlog (Part 3) — and still not a single line of Python. Today we fix that. Four commands form the heart of mini: propose, break down, build, close. One turn of the loop = one phase = one commit. next — propose mini next next is the entry point of the loop, and th