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bin/gems/rouge-4.5.1/lib/rouge/demos/isabelle

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+theory demo imports Main begin
+
+section ‹Inductive predicates for lists›
+
+datatype 'a list = Nil ("[]") | Cons 'a "'a list" ("_ # _")
+
+fun length :: "'a list ⇒ nat" where
+  "length [] = 0" | "length (x # xs) = 1 + length xs"
+
+inductive ζ :: "'a list ⇒ nat ⇒ bool" where
+Nil[intro!]:  "ζ [] 0" |
+Cons[intro]: "ζ xs l ⟹ ζ (x # xs) (1 + l)"
+
+(* Not the answer? *)
+lemma "ζ xs 42"
+  oops