Cher Stéfan & tous,

> Beware: I'm talking about make-variable-buffer-local, which is
> different (but I think it's what you want here).

Yes, this is what I meant, but I use this function so seldom that I
mispelled it, and as I am my own spell-checker...

Ok, I removed the function list and used a macro 5x5-defvar-local
instead according to your kind recommendations.

I also added a final message "5x5 Solution computation done." ---
hopefully this is correct English --- to mask the obscure Calc verbiage.

I also realized that there was a bug in 5x5, when making random grids,
it was sometimes impossible for the solver to find a solution, and that
happened while I was demonstrating the solver to my 6 years old son ---
then I had to explained the reason for that kind of failure, which was
incredibly more difficult than understand it and find a correction for
it...

I realized that the random grid generator was indeed making a grid whose
each position is random, rather than making a set of random 5x5
moves. The former does not ensure the existence of a solution, while the
latter does. I don't know if that was the initial intention to have this
behaviour, but as « à l'impossible nul n'est tenu », I considered it a
bug, and I corrected it along with providing the solver. So my
contribution is 3 fold:

I   Add an arithmetic solver

II  Make 5x5 multisession

III Ensure that random grids always have a solution --- this is a quick
    patch that makes it only when the grid size is 5, but I am not sure
    that that would work for other size, as I have not proved it
    mathematically.

Here is the updated Changelog, I commonalized comment when possible.

-----------------------------------------------------------------------
2011-05-22  Vincent Belaïche  <vincentb1@users.sourceforge.net>

	* play/5x5.el: I/ Add an arithmetic solver to suggest positions to
	click on. II/ Make 5x5 multisession. III/ Ensure that random grids
	always have a solution in grid size = 5 cases
	(5x5-mode-map): Add keybinding to function `5x5-solve-suggest'.
	(5x5-solver-output): New defvar, output of function
	`5x5-solve-suggest'.
	(5x5-grid, 5x5-x-pos, 5x5-y-pos, 5x5-moves, 5x5-cracking): Make
	these variables buffer local to achieve 5x5 multi-session-ness.
	(5x5): Set 5x5-grid-size only if SIZE is non-negative.
	(5x5-grid-to-vec): New defun. Convert a 5x5 game grid into a Calc
	matrix in Z/2Z.
	(5x5-vec-to-grid): New defun. Convert a Calc matrix in Z/2Z into a
	5x5 game grid.
	(5x5-log-buffer): New defvar. Not defined, provisionned for
	debugging Arithmetric solver.
	(5x5-log-init): New defun. Defined to dummy, provisionned for
	debugging Arithmetric solver.
	(5x5-log): New defun. Defined to dummy, provisionned for debugging
	Arithmetric solver.
	(5x5-solver): New defun, make the actual algorithm of arithmetic
	solver, to be usable that function needs some wrapper to the 5x5
	user interace, this wapper is function `5x5-solve-suggest' for
	invocation, and function `5x5-draw-grid' for the diplay.
	(5x5-solve-suggest): New defun, invoke the arithmetic solver, and
	display solution.
	(5x5-randomize): Use 5x5-make-move instead of 5x5-flip-cell to
	randomize a grid so that we ensure that there is always a
	solution.
	(5x5-make-random-grid) allow other movement than flipping.
-----------------------------------------------------------------------

VBR,
   Vincent