I've recently posted another question regarding natural deduction proofs and I've definitely made some progress, but I'm now stuck with a proof which seems like it could be flawed.
Now as you can see, it looks like I've got it all figured out, however you can see an error is returned for incorrect use of negation introduction. Now there seems to be a contradiction in the premises on lines 4 and 5: as per lines 9 and 10, R is true and P is false. I went with P being false (line 10) which leads to a contradiction, seemingly making the proof work out. However, I could just as well have gone with R being true (line 9), which, according to line 5, would not prove my contradiction as I must prove Q.
Am I missing something obvious here or do you think the proof is broken?