3436 - experiment: APL-style prompt

(https://www.youtube.com/watch?v=DsZdfnlh_d0)

It works, kinda, but fails manual tests VIII, X and XII. So even ironing
out extra-peek bugs isn't enough to provide a prompt.

And then there's the fact that there's always some way to make program
output look like programmer input..
3437 - cleanup