Inside a TikZ picture, most symbols need to have the category code 12 (normal text) in order to ensure that
the parser works properly. This is typically not the case when packages like babel are used, which change
catcodes aggressively.
To solve this problem, TikZ provides a small library also called babel (which can, however, also be used
together with any other package that globally changes category codes). What it does is to reset the category
codes at the beginning of every {tikzpicture} and to restore them at the beginning of every node. In
almost all cases, this is exactly would you would expect and need, so I recommend to always load this library
by saying \usetikzlibrary{babel}.
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