On Thursday, March 10, 2016 at 8:17:47 AM UTC+1, Jeroen Demeyer wrote:
>
> On 2016-03-09 23:56, Thierry wrote: 
> > Willing to 
> > change the behaviour will be similar to requesting that 'NaN in RR' 
> should 
> > return False. 
> Not the topic of this thread, but I really think that "NaN in RR" should 
> return False. RR represents the real numbers and NaN is *not a number*. 


It's not that there were no attempts to fix the Parent.__contains__ function
by not depending on bool(a == b) --- this expression seems to have a
strange attraction to many, and is used in wildly different contexts

http://trac.sagemath.org/ticket/17984

Note that Sage usually defines "x in P" as "P(x) == x" which would 
> actually give False for NaN since NaN != NaN. 
>

Wrong. NaN is even a named constant.

sage: NaN is NaN
True

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