Isabelle language - fixed lexer and added sample

Also, Isabelle is very polular in academia.
See for example http://scholar.google.de/scholar?q=isabelle%2FHOL

In around 40 days, the seL4 microkernel [1] with its
Isabelle proofs is (probably) released on github [2].
[1] http://sel4.systems/
[2] https://lists.cam.ac.uk/mailman/htdig/cl-isabelle-users/2014-June/msg00011.html

samples/Isabelle/HelloWorld.thy

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+theory HelloWorld
+imports Main
+begin
+
+section{*Playing around with Isabelle*}
+
+text{* creating a lemma with the name hello_world*}
+lemma hello_world: "True" by simp
+
+(*inspecting it*)
+thm hello_world
+
+text{* defining a string constant HelloWorld *}
+
+definition HelloWorld :: "string" where
+  "HelloWorld \<equiv> ''Hello World!''"
+
+(*reversing HelloWorld twice yilds HelloWorld again*)
+theorem "rev (rev HelloWorld) = HelloWorld"
+  by (fact List.rev_rev_ident)
+
+text{*now we delete the already proven List.rev_rev_ident lema and show it by hand*}
+declare List.rev_rev_ident[simp del]
+hide_fact List.rev_rev_ident
+
+(*It's trivial since we can just 'execute' it*)
+corollary "rev (rev HelloWorld) = HelloWorld"
+  apply(simp add: HelloWorld_def)
+  done
+
+text{*does it hold in general?*}
+theorem rev_rev_ident:"rev (rev l) = l"
+  proof(induction l)
+  case Nil thus ?case by simp
+  next
+  case (Cons l ls)
+    assume IH: "rev (rev ls) = ls"
+    have "rev (l#ls) = (rev ls) @ [l]" by simp
+    hence "rev (rev (l#ls)) = rev ((rev ls) @ [l])" by simp
+    also have "\<dots> = [l] @ rev (rev ls)" by simp
+    finally show "rev (rev (l#ls)) = l#ls" using IH by simp
+  qed
+
+corollary "\<forall>(l::string). rev (rev l) = l" by(fastforce intro: rev_rev_ident)
+
+end