Exercise 2: -r, -x and -d -r Against an Attached Session — Possible Solution
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Setup: terminal A runs `screen -S demo` and stays attached.

1. In terminal B: screen -r demo
   Screen refuses. It reports that there is a screen on
   "12345.demo (Attached)" and that there is no screen to be resumed.
   Terminal A is unaffected. -r only resumes DETACHED sessions.

2. In terminal B: screen -x demo
   Terminal B attaches as well. Both terminals now show the same
   window: type in either one and the text appears in both. Terminal
   A is unaffected apart from seeing B's typing. Detach B with
   Ctrl-a d when done.

3. In terminal B: screen -d -r demo
   Screen detaches the session from terminal A (A returns to its
   normal shell with a "[remote detached ...]"-style message), then
   attaches it to terminal B. Only B is now attached.

WHY THIS WORKS AS AN ANSWER
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It shows the three different behaviours side by side: -r refuses an
attached session, -x shares it, and -d -r takes it over. The exact
wording of screen's messages varies slightly between versions.