diff --git a/src/Pure/term.scala b/src/Pure/term.scala
--- a/src/Pure/term.scala
+++ b/src/Pure/term.scala
@@ -1,221 +1,228 @@
 /*  Title:      Pure/term.scala
     Author:     Makarius
 
 Lambda terms, types, sorts.
 
 Note: Isabelle/ML is the primary environment for logical operations.
 */
 
 package isabelle
 
 
 object Term
 {
   /* types and terms */
 
   sealed case class Indexname(name: String, index: Int = 0)
   {
     override def toString: String =
       if (index == -1) name
       else {
         val dot =
           Symbol.explode(name).reverse match {
             case _ :: s :: _ if s == Symbol.sub_decoded || s == Symbol.sub => false
             case c :: _ => Symbol.is_digit(c)
             case _ => true
           }
         if (dot) "?" + name + "." + index
         else if (index != 0) "?" + name + index
         else "?" + name
       }
   }
 
   type Class = String
   type Sort = List[Class]
 
   sealed abstract class Typ
   case class Type(name: String, args: List[Typ] = Nil) extends Typ
   {
     override def toString: String =
       if (this == dummyT) "_"
       else "Type(" + name + (if (args.isEmpty) "" else "," + args) + ")"
   }
   case class TFree(name: String, sort: Sort = Nil) extends Typ
   {
     override def toString: String =
       "TFree(" + name + (if (sort.isEmpty) "" else "," + sort) + ")"
   }
   case class TVar(name: Indexname, sort: Sort = Nil) extends Typ
   {
     override def toString: String =
       "TVar(" + name + (if (sort.isEmpty) "" else "," + sort) + ")"
   }
   val dummyT = Type("dummy")
 
   sealed abstract class Term
+  {
+    def head: Term =
+      this match {
+        case App(fun, _) => fun.head
+        case _ => this
+      }
+  }
   case class Const(name: String, typargs: List[Typ] = Nil) extends Term
   {
     override def toString: String =
       if (this == dummy) "_"
       else "Const(" + name + (if (typargs.isEmpty) "" else "," + typargs) + ")"
   }
   case class Free(name: String, typ: Typ = dummyT) extends Term
   {
     override def toString: String =
       "Free(" + name + (if (typ == dummyT) "" else "," + typ) + ")"
   }
   case class Var(name: Indexname, typ: Typ = dummyT) extends Term
   {
     override def toString: String =
       "Var(" + name + (if (typ == dummyT) "" else "," + typ) + ")"
   }
   case class Bound(index: Int) extends Term
   case class Abs(name: String, typ: Typ, body: Term) extends Term
   case class App(fun: Term, arg: Term) extends Term
   {
     override def toString: String =
       this match {
         case OFCLASS(ty, c) => "OFCLASS(" + ty + "," + c + ")"
         case _ => "App(" + fun + "," + arg + ")"
       }
   }
 
   def dummy_pattern(ty: Typ): Term = Const("Pure.dummy_pattern", List(ty))
   val dummy: Term = dummy_pattern(dummyT)
 
   sealed abstract class Proof
   case object MinProof extends Proof
   case class PBound(index: Int) extends Proof
   case class Abst(name: String, typ: Typ, body: Proof) extends Proof
   case class AbsP(name: String, hyp: Term, body: Proof) extends Proof
   case class Appt(fun: Proof, arg: Term) extends Proof
   case class AppP(fun: Proof, arg: Proof) extends Proof
   case class Hyp(hyp: Term) extends Proof
   case class PAxm(name: String, types: List[Typ]) extends Proof
   case class OfClass(typ: Typ, cls: Class) extends Proof
   case class Oracle(name: String, prop: Term, types: List[Typ]) extends Proof
   case class PThm(serial: Long, theory_name: String, name: String, types: List[Typ]) extends Proof
 
 
   /* type classes within the logic */
 
   object Class_Const
   {
     val suffix = "_class"
     def apply(c: Class): String = c + suffix
     def unapply(s: String): Option[Class] =
       if (s.endsWith(suffix)) Some(s.substring(0, s.length - suffix.length)) else None
   }
 
   object OFCLASS
   {
     def apply(ty: Typ, s: Sort): List[Term] = s.map(c => apply(ty, c))
 
     def apply(ty: Typ, c: Class): Term =
       App(Const(Class_Const(c), List(ty)), Const(Pure_Thy.TYPE, List(ty)))
 
     def unapply(t: Term): Option[(Typ, String)] =
       t match {
         case App(Const(Class_Const(c), List(ty)), Const(Pure_Thy.TYPE, List(ty1)))
         if ty == ty1 => Some((ty, c))
         case _ => None
       }
   }
 
 
 
   /** cache **/
 
   def make_cache(initial_size: Int = 131071, max_string: Int = Integer.MAX_VALUE): Cache =
     new Cache(initial_size, max_string)
 
   class Cache private[Term](initial_size: Int, max_string: Int)
     extends isabelle.Cache(initial_size, max_string)
   {
     protected def cache_indexname(x: Indexname): Indexname =
       lookup(x) getOrElse store(Indexname(cache_string(x.name), x.index))
 
     protected def cache_sort(x: Sort): Sort =
       if (x.isEmpty) Nil else lookup(x) getOrElse store(x.map(cache_string(_)))
 
     protected def cache_typ(x: Typ): Typ =
     {
       if (x == dummyT) dummyT
       else
         lookup(x) match {
           case Some(y) => y
           case None =>
             x match {
               case Type(name, args) => store(Type(cache_string(name), cache_typs(args)))
               case TFree(name, sort) => store(TFree(cache_string(name), cache_sort(sort)))
               case TVar(name, sort) => store(TVar(cache_indexname(name), cache_sort(sort)))
             }
         }
     }
 
     protected def cache_typs(x: List[Typ]): List[Typ] =
     {
       if (x.isEmpty) Nil
       else {
         lookup(x) match {
           case Some(y) => y
           case None => store(x.map(cache_typ(_)))
         }
       }
     }
 
     protected def cache_term(x: Term): Term =
     {
       lookup(x) match {
         case Some(y) => y
         case None =>
           x match {
             case Const(name, typargs) => store(Const(cache_string(name), cache_typs(typargs)))
             case Free(name, typ) => store(Free(cache_string(name), cache_typ(typ)))
             case Var(name, typ) => store(Var(cache_indexname(name), cache_typ(typ)))
             case Bound(_) => store(x)
             case Abs(name, typ, body) =>
               store(Abs(cache_string(name), cache_typ(typ), cache_term(body)))
             case App(fun, arg) => store(App(cache_term(fun), cache_term(arg)))
           }
       }
     }
 
     protected def cache_proof(x: Proof): Proof =
     {
       if (x == MinProof) MinProof
       else {
         lookup(x) match {
           case Some(y) => y
           case None =>
             x match {
               case PBound(_) => store(x)
               case Abst(name, typ, body) =>
                 store(Abst(cache_string(name), cache_typ(typ), cache_proof(body)))
               case AbsP(name, hyp, body) =>
                 store(AbsP(cache_string(name), cache_term(hyp), cache_proof(body)))
               case Appt(fun, arg) => store(Appt(cache_proof(fun), cache_term(arg)))
               case AppP(fun, arg) => store(AppP(cache_proof(fun), cache_proof(arg)))
               case Hyp(hyp) => store(Hyp(cache_term(hyp)))
               case PAxm(name, types) => store(PAxm(cache_string(name), cache_typs(types)))
               case OfClass(typ, cls) => store(OfClass(cache_typ(typ), cache_string(cls)))
               case Oracle(name, prop, types) =>
                 store(Oracle(cache_string(name), cache_term(prop), cache_typs(types)))
               case PThm(serial, theory_name, name, types) =>
                 store(PThm(serial, cache_string(theory_name), cache_string(name), cache_typs(types)))
             }
         }
       }
     }
 
     // main methods
     def indexname(x: Indexname): Indexname = synchronized { cache_indexname(x) }
     def sort(x: Sort): Sort = synchronized { cache_sort(x) }
     def typ(x: Typ): Typ = synchronized { cache_typ(x) }
     def term(x: Term): Term = synchronized { cache_term(x) }
     def proof(x: Proof): Proof = synchronized { cache_proof(x) }
 
     def position(x: Position.T): Position.T =
       synchronized { x.map({ case (a, b) => (cache_string(a), cache_string(b)) }) }
   }
 }