Re: [isabelle] Data‑types: How to use discriminators?

Hi Yannik,

Yes, that was the intent. I though this was the same as to say there exist an “a” such
that “t = α a”. Or else, this just mean nothing, unless “a” is fixed and in which case
this is indeed not provable. Is “a” implicitly fixed in the proposition I tested?
Yes, fresh variables in theorem statements are implicitly fixed. You can also specify variables that you can instantiate later, as in

schematic_lemma "isα t ==> t = α ?a"

but this does not work well with Isar proofs, parallelisation of proof checking, and (in this case) proof automation. So it's better to explicitly use an existential quantifier here.


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