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I am currently using the book Language, Proof and Logic

So far I have 1. a=c v b=c

subproof 2. a=c =intro 3 c=c =elim 1,2 4. c=a =elim 2,3

end subproof

new subproof 5. b=c 6. c=c =intro 7. c=b =elim 5,6

and when i end this subproof, and put down: 8. c=a v b=c velim 1, 2-4, 5-6

it is invalid? I am not sure how to approach this

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    Does the problem prohibit using symmetry of equality?
    – commando
    May 11 '14 at 18:39
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It's certainly valid. Your derivation doesn't seem exactly right, but it can be modified to become correct.

enter image description here

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  • Is this some kind of software package? Which is it? (Just curious.)
    – user3164
    May 11 '14 at 18:50
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    It's from the old edition of Language, Proof, and Logic. Barwise & Etchemendy have named it Fitch. (You have to admit, it looks much sexier than the new version that you can see in the modified question.) May 11 '14 at 18:56

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