From: David Tran <email55555@...>
Date: 2005-08-25T00:27:09+09:00
Subject: Re: [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.