Re: [isabelle] On Recursion Induction

On Fri, May 11, 2012 at 6:59 AM, Christian Sternagel
<c-sterna at> wrote:
> Since I was not able to find any rule that directly reduces a meta-equality
> into two implications with
>  find_theorems "(?A ==> ?B) ==> (?B ==> ?A) ==> ?A == ?B"
> but such a rule is obviously applied. This step is somewhat "magic" (I guess
> it is part of the Isabelle/Isar/Pure framework).

You used a too-specific pattern with your theorem search. A term like
"?A ==> ?B" is parsed as "Trueprop ?A ==> Trueprop ?B", with ?A and ?B
of type "bool". Of course you want ?A and ?B to be parsed as type
"prop" here, so you need to add "PROP" tags to tell the parser what
you want:

find_theorems "(PROP ?A ==> PROP ?B) ==> (PROP ?B ==> PROP ?A) ==>

found 1 theorem(s):

  [| PROP ?phi ==> PROP ?psi; PROP ?psi ==> PROP ?phi |] ==> PROP ?phi
== PROP ?psi

If there is any "magic" it is in the parser, which magically inserts
"Trueprop" everywhere unless you tell it not to. ;)

- Brian

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