Use ruby-debug19 instead of ruby-debug.
Typographical changes for coding style.
Set#pick was not really useful.
More typographical changes.
Coding style: better formatting, removed one useless return value.
Sticking to ‘debugger’ after all.
Correcting one change too much.
Added test for guess method to specs.
Be verbose in :guess mode.
Wording in messages.
Renaming ‘to_s’ to ‘display’ for Cell and Grid, otherwise it affects inspect.
Document one task ahead.
WIP.  Restoring “place_single” methods to place individual values in cells.  First step.
End of all the nonsense.  place_single now does what it should.
Much simpler implementation of place_singles.
Plugged place_single back in.  It doesn’t make the solver noticeably faster.
Added support for place_singles option in command line interface.
Moved place_singles call to Group.
That wasn’t needed actually.
Propagating constraint from place_single.  Still no noticeable speed improvement.