src/Pure/term_xml.scala

         { case (Nil, a) => val (b, c) = pair(proof, term)(a); Appt(b, c) },
         { case (Nil, a) => val (b, c) = pair(proof, proof)(a); AppP(b, c) },
         { case (Nil, a) => Hyp(term(a)) },
         { case (List(a), b) => PAxm(a, list(typ)(b)) },
         { case (List(a), b) => OfClass(typ(b), a) },
-        { case (List(a), b) => val (c, d) = pair(term, list(typ))(b); Oracle(a, c, d) },
+        { case (List(a), b) => val (c, d) = pair(option(term), list(typ))(b); Oracle(a, c, d) },
         { case (List(a, b, c), d) => PThm(long_atom(a), b, c, list(typ)(d)) }))
   }
 }