to the HTML on-line version of the standard library

This allows (generalize->split->introduce) to stop
when the split transformation does not make progress.

select_iter changes the cursor used by the goals_view for changing nodes
with keyboard arrow keys. selection#select_iter should be avoided alone.

Computer division for master

See merge request !5

        without an additional assert and splitting. The splitting make
        also the bench faster by 2sec.

See #151.
At the moment, this transformation is only integrated into TryWhy3,
in order to restore left-hand side splitting under vc:sp.

I suggest to integrate "generalize" into split_vc unconditionally.
I've tested that, the sessions were only broken for 4 examples:
  examples/tortoise_and_hare/
  examples/verifythis_2018_array_based_queuing_lock_2/
  examples/verifythis_2018_mind_the_gap_2/
  examples/multiprecision/toom/