Spanx or similar...

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I have purchased a very unforgiving dress for a 'do' and will need underwear to be invisible and helpful .

Question 1 - how do you go for a wee?

Question 2 - do the cycling shorts type fandangos roll down at the top?

I got some from primark the other week as I had a very clingy dress for a birthday, only £4 and looked great

Oh by the way they were fine going to the toilet and didn't roll up