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Please feel free to change the title of this question since I'm not sure if it's fitting.

For example, I'd like to draw a rule with the same height as the thickness of a frame line of a standard tcolorbox.

I had a look at the tcolorbox package code, and I assume that the line leftrule/.store in=\kvtcb@left@rule is somehow essential for what I want. But if I try






it doesn't work and LaTeX returns the error ! Undefined control sequence..

So my more general question is: How can I access package internal values like this to use them in my document?

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Please further specify what you mean by "it doesn't work". – Jubobs Jun 29 '14 at 16:37
You need \makeatletter before \rule and \makeatother after the closing brace. There is no general answer, I'm afraid: it much depends on what internal value you need access to. – egreg Jun 29 '14 at 16:38
@Jubobs I edited my question. – MaxD Jun 29 '14 at 16:42
Related: – MaxD Jun 29 '14 at 16:48

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