I find myself with a sea of conversions of the form exhibited by `filter_empty_conv`. `HOL.List` is missing its obvious friend:
shows "( = filter P xs) = (\<forall>x\<in>set xs. \<not> P x)"
by (induct xs) simp_all
which could also be derived via a rule of the form:
(x = y) = z ==> (y = x) = z
(and so forth for other symmetric or commutative operators such as inf and sup).
One could imagine defining an attribute `symconv` like `symmetric` to handle this:
lemmas empty_filter_conv = filter_empty_conv[symconv]
But in my ideal world I wouldn’t need to type this out and make up another name, but would instead bind both theorems to the same name, e.g.
shows "( = filter P xs) = (\<forall>x\<in>set xs. \<not> P x)”
> ( = filter P xs) = (\<forall>x\<in>set xs. \<not> P x)
> (filter P xs = ) = (\<forall>x\<in>set xs. \<not> P x)
However AIUI attributes map a single theorem to a single theorem, so this isn’t going to work.
All I can think of is to create another keyword like `lemma` that does a bit of post-processing before the binding is made. This feels a bit heavyweight.
Has anyone got a better solution? It might help to make these sorts of lemma pairs more systematic in the distribution.