Why are ranges not possible in display filter frame.number?

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I want to use a display filter to only show a certain range of frames. What works:

frame.number >= 20 && frame.number <= 30

I tried the more user friendly way with ranges, which does not work:

frame.number in { 20..30 }

Why is that? According to the user guide I would have expected the range notation to work with simple numbers like that.

(I tried with Version 2.2.6 on Debian 9 GNU/Linux and MS Windows 7 and with v2.4.5 also on Windows 7.)