src/Pure/Thy/export_theory.scala

   {
     val TYPE = Value("type")
     val CONST = Value("const")
     val AXIOM = Value("axiom")
     val THM = Value("thm")
+    val PROOF = Value("proof")
     val CLASS = Value("class")
     val LOCALE = Value("locale")
     val LOCALE_DEPENDENCY = Value("locale_dependency")
   }