[QUIZ] Sodoku Solver (#43)

Kroeger Simon wrote:
...
BTW a lot of speed comes from rather minor changes, like changing

a_set -= [a_value]

to

a_set.delete(a_value)

this makes a HUGE difference. This is obvious if you take some time
to think about it, but it would be realy nice to optimize operators
like -= where you have no chance to use the intermediate objects anyway.

Thanks for the hint, unfortunately, I cannot use it on my last program.
My last program use dup to duplicate possibilities hash,
it is shallow copy, so the
element -= [ a_value ] will not impact to origin copied data
but
element.delete( a_value ) will modify my origin copied data.