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I'm writing some math proofs. I want to have the possibility to "expand" some text in the final document if the user needs further information.

For example I have
http://i53.tinypic.com/r0e653.png
and after I click on the blue arrow I want to obtain
http://i55.tinypic.com/aahbbo.png

Is it ...

I had in mind something like an "if-then" construction ("if math environment do math bold, if text environment do text bold"), but yoriz solution is just perfect for me.

Thanks to all.

How can I define one new command to have the text formatted in bold both in math and in text mode?