challenge -- correction of Hans' model

[From Bill Powers (950928.0900 MDT)]

Hans Blom (950927) --

I found that your program was running into computational oscillations
for some lower values of the slowing factor. This was apparently cured
by making the corrections of pmm and pnn slower:

  {estimate of m and n noise variances}
  pmm := pmm + (sqr (k - kold) - pmm) / 100.0; kold := k;
  pnn := pnn + (sqr (d - dold) - pnn) / 100.0; dold := d;

[dividing by 100 instead of 20]

With that change, your model controls almost exactly as well as mine at
the lower values of "slow" as well as the higher values.

···

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Best,

Bill P.