(*
      Example proof document for Isabelle/Isar Proof General.
   
      Example.thy,v 9.1 2008/03/14 20:42:52 makarius Exp
*)

theory Example imports Main begin

text {* Proper proof text -- \textit{naive version}. *}

theorem and_comms: "A & B --> B & A"
proof
  assume "A & B"
  then show "B & A"
  proof
    assume B and A
    then show ?thesis ..
 qed
qed

text {* Proper proof text -- \textit{advanced version}. *}

theorem "A & B --> B & A"
proof
  assume "A & B"
  then obtain B and A ..
  then show "B & A" ..
qed


text {* Unstructured proof script. *}

theorem "A & B --> B & A"
  apply (rule impI)
  apply (erule conjE)
  apply (rule conjI)
  apply assumption
  apply assumption
done

end