--- a/progs/token.scala	Sun Oct 27 14:17:55 2013 +0000
+++ b/progs/token.scala	Sun Oct 27 15:16:42 2013 +0000
@@ -89,40 +89,38 @@
   case STAR(r) => SEQ(der(c, r), STAR(r))
 }
 
-// returns the position of the last Some-element in the list
-def last(stack: List[Option[Rexp]]) : Int = stack match {
-  case Nil => 0
-  case None::stack => last(stack)
-  case Some(r)::stack => 1 + stack.length
-}
 
 // calculates derivatives until all of them are zeroable
 @tailrec
-def munch(cs: List[Char], rs: List[Rexp], stack: List[Option[Rexp]]) : Int = (cs, rs) match {
-  case (_, Nil) => last(stack)
-  case (Nil, _) => last(stack)
-  case (c::cs, rs) => {
-    val ds = rs.map(der(c, _))
-    val rs_nzero = ds.filterNot(zeroable(_))
-    val rs_nulls = ds.filter(nullable(_))
-    val opt = Try(Some(rs_nulls.head)) getOrElse None
-    munch(cs, rs_nzero, opt::stack)
+def munch(s: List[Char], 
+          pos: Int, 
+          rs: List[Rexp], 
+          last: Option[Int]): Option[Int] = rs match {
+  case Nil => last
+  case rs if (s.length <= pos) => last
+  case rs => {
+    val ders = rs.map(der(s(pos), _))
+    val rs_nzero = ders.filterNot(zeroable(_))
+    val rs_nulls = ders.filter(nullable(_))
+    val new_last = if (rs_nulls != Nil) Some(pos) else last
+    munch(s, 1 + pos, rs_nzero, new_last)
   }
 }
 
 // iterates the munching function and prints 
 // out the component strings
 @tailrec
-def tokenize(s: String, rs: List[Rexp]) : Unit = munch(s.toList, rs, Nil) match {
-  case 0 if (s == "") => println("EOF")
-  case 0 => println(s"Lexing error: $s")
-  case n => {
-    val (head, tail) = s.splitAt(n)
-    print(s"|${head.replaceAll("\n","Ret")}|")
+def tokenize(s: String, rs: List[Rexp]) : Unit = munch(s.toList, 0, rs, None) match {
+  case None if (s == "") => println("EOF")
+  case None => println(s"Lexing error: $s")
+  case Some(n) => {
+    val (head, tail) = s.splitAt(n + 1)
+    print(s"|${head.replaceAll("\n","RET")}|")
     tokenize(tail, rs)
   }
 }
 
+
 val test_prog = """
 start := XXX;
 x := start;