Open pool or closed flask

A pool of molecules gains a supply each minute and loses a share of itself. You will write the rule for one minute, repeat it for many minutes, and compare a cell, which is fed all the time, with a closed flask, which is not.

The idea. A loop can repeat a rule. If the supply goes on, the pool settles where what is used equals what is supplied. If the supply stops, the pool runs down.

Your tasks.

1. In next_pool, return the pool a minute later: it gains the supply and loses a share of itself.
2. In run, repeat next_pool for the given number of minutes and return the final pool.
3. Set steady to the pool that the cell settles at, worked out from the supply and the share.