replaced apply-style proof for instance Multiset :: plus_ac0 by recommended Isar proof style
theory FP0 = PreList:
datatype 'a list = Nil ("[]")
| Cons 'a "'a list" (infixr "#" 65)
consts app :: "'a list \<Rightarrow> 'a list \<Rightarrow> 'a list" (infixr "@" 65)
rev :: "'a list \<Rightarrow> 'a list"
primrec
"[] @ ys = ys"
"(x # xs) @ ys = x # (xs @ ys)"
primrec
"rev [] = []"
"rev (x # xs) = (rev xs) @ (x # [])"
theorem rev_rev [simp]: "rev(rev xs) = xs"
(*<*)oops(*>*)text_raw{*\isanewline\isanewline*}
end