diff -r b88670c5d04b -r 86b2aeae3e98 progs/token.scala
--- a/progs/token.scala	Fri Oct 16 08:56:25 2015 +0100
+++ b/progs/token.scala	Fri Oct 16 14:27:20 2015 +0100
@@ -12,9 +12,9 @@
 case class RECD(x: String, r: Rexp) extends Rexp
 
 abstract class Val
-case object Void extends Val
+case object Empty extends Val
 case class Chr(c: Char) extends Val
-case class Sequ(v1: Val, v2: Val) extends Val
+case class Seq(v1: Val, v2: Val) extends Val
 case class Left(v: Val) extends Val
 case class Right(v: Val) extends Val
 case class Stars(vs: List[Val]) extends Val
@@ -76,45 +76,45 @@
 
 // extracts a string from value
 def flatten(v: Val) : String = v match {
-  case Void => ""
+  case Empty => ""
   case Chr(c) => c.toString
   case Left(v) => flatten(v)
   case Right(v) => flatten(v)
-  case Sequ(v1, v2) => flatten(v1) + flatten(v2)
+  case Seq(v1, v2) => flatten(v1) + flatten(v2)
   case Stars(vs) => vs.map(flatten).mkString
   case Rec(_, v) => flatten(v)
 }
 
 // extracts an environment from a value
 def env(v: Val) : List[(String, String)] = v match {
-  case Void => Nil
+  case Empty => Nil
   case Chr(c) => Nil
   case Left(v) => env(v)
   case Right(v) => env(v)
-  case Sequ(v1, v2) => env(v1) ::: env(v2)
+  case Seq(v1, v2) => env(v1) ::: env(v2)
   case Stars(vs) => vs.flatMap(env)
   case Rec(x, v) => (x, flatten(v))::env(v)
 }
 
 // injection part
 def mkeps(r: Rexp) : Val = r match {
-  case EMPTY => Void
+  case EMPTY => Empty
   case ALT(r1, r2) => 
     if (nullable(r1)) Left(mkeps(r1)) else Right(mkeps(r2))
-  case SEQ(r1, r2) => Sequ(mkeps(r1), mkeps(r2))
+  case SEQ(r1, r2) => Seq(mkeps(r1), mkeps(r2))
   case STAR(r) => Stars(Nil)
   case RECD(x, r) => Rec(x, mkeps(r))
 }
 
 
 def inj(r: Rexp, c: Char, v: Val) : Val = (r, v) match {
-  case (STAR(r), Sequ(v1, Stars(vs))) => Stars(inj(r, c, v1)::vs)
-  case (SEQ(r1, r2), Sequ(v1, v2)) => Sequ(inj(r1, c, v1), v2)
-  case (SEQ(r1, r2), Left(Sequ(v1, v2))) => Sequ(inj(r1, c, v1), v2)
-  case (SEQ(r1, r2), Right(v2)) => Sequ(mkeps(r1), inj(r2, c, v2))
+  case (STAR(r), Seq(v1, Stars(vs))) => Stars(inj(r, c, v1)::vs)
+  case (SEQ(r1, r2), Seq(v1, v2)) => Seq(inj(r1, c, v1), v2)
+  case (SEQ(r1, r2), Left(Seq(v1, v2))) => Seq(inj(r1, c, v1), v2)
+  case (SEQ(r1, r2), Right(v2)) => Seq(mkeps(r1), inj(r2, c, v2))
   case (ALT(r1, r2), Left(v1)) => Left(inj(r1, c, v1))
   case (ALT(r1, r2), Right(v2)) => Right(inj(r2, c, v2))
-  case (CHAR(d), Void) => Chr(c) 
+  case (CHAR(d), Empty) => Chr(c) 
   case (RECD(x, r1), _) => Rec(x, inj(r1, c, v))
 }
 
@@ -126,6 +126,8 @@
 
 def lexing(r: Rexp, s: String) : Val = lex(r, s.toList)
 
+lexing(("ab" | "ab") ~ ("b" | EMPTY), "ab")
+
 // some "rectification" functions for simplification
 def F_ID(v: Val): Val = v
 def F_RIGHT(f: Val => Val) = (v:Val) => Right(f(v))
@@ -135,10 +137,12 @@
   case Left(v) => Left(f1(v))
 }
 def F_SEQ(f1: Val => Val, f2: Val => Val) = (v:Val) => v match {
-  case Sequ(v1, v2) => Sequ(f1(v1), f2(v2))
+  case Seq(v1, v2) => Seq(f1(v1), f2(v2))
 }
-def F_SEQ_Void1(f1: Val => Val, f2: Val => Val) = (v:Val) => Sequ(f1(Void), f2(v))
-def F_SEQ_Void2(f1: Val => Val, f2: Val => Val) = (v:Val) => Sequ(f1(v), f2(Void))
+def F_SEQ_Empty1(f1: Val => Val, f2: Val => Val) = 
+  (v:Val) => Seq(f1(Empty), f2(v))
+def F_SEQ_Empty2(f1: Val => Val, f2: Val => Val) = 
+  (v:Val) => Seq(f1(v), f2(Empty))
 def F_RECD(f: Val => Val) = (v:Val) => v match {
   case Rec(x, v) => Rec(x, f(v))
 }
@@ -163,8 +167,8 @@
     (r1s, r2s) match {
       case (NULL, _) => (NULL, F_ERROR)
       case (_, NULL) => (NULL, F_ERROR)
-      case (EMPTY, _) => (r2s, F_SEQ_Void1(f1s, f2s))
-      case (_, EMPTY) => (r1s, F_SEQ_Void2(f1s, f2s))
+      case (EMPTY, _) => (r2s, F_SEQ_Empty1(f1s, f2s))
+      case (_, EMPTY) => (r1s, F_SEQ_Empty2(f1s, f2s))
       case _ => (SEQ(r1s,r2s), F_SEQ(f1s, f2s))
     }
   }
@@ -185,7 +189,7 @@
 
 def lexing_simp(r: Rexp, s: String) : Val = lex_simp(r, s.toList)
 
-lexing_simp(("a" + "ab") | ("b" + ""), "ab")
+lexing_simp(("a" | "ab") ~ ("b" | ""), "ab")
 
 // Lexing Rules for a Small While Language