In this question here the OP asks for hints for a problem rather than a full proof.

http://math.stackexchange.com/questions/14477

Now, while I would like to respect that request, I also feel that questions on this site are not intended just for the OP's benefit.  This leads me to the question...

> **Question**: Is there any way to use some form of *spoiler space*, so that it's possible to post the answer for the other readers' benefit, but at the same time hiding it from those who do not want it?

My attempted "look at the previous version of this post" turned out a disaster.  I've seen people use rot13, but that seems like a lot of fuss (and clashes with the mathematics).

On some sites they use white text on white background for spoily material, which, when you select with the mouse, reveals the text.  Is that possible?

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Testing:

>! Spoiler Space

>! More spoiler space

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>! Spoiler Space
>! More spoiler space

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>! What happens if I write a really long sentence.  Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test

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>! Maybe it'll involve some maths like $E=mc^2$ or exclamation marks $n!=n \times (n-1)!$.