diff -r c9027db225cc -r 6a43ea9305ba progs/scala/re2.scala
--- a/progs/scala/re2.scala	Fri Mar 18 15:03:54 2016 +0000
+++ b/progs/scala/re2.scala	Sat Mar 19 23:27:29 2016 +0000
@@ -3,8 +3,8 @@
 import scala.annotation.tailrec   
 
 abstract class Rexp 
-case object NULL extends Rexp
-case object EMPTY extends Rexp
+case object ZERO extends Rexp
+case object ONE extends Rexp
 case class CHAR(c: Char) extends Rexp
 case class ALT(r1: Rexp, r2: Rexp) extends Rexp 
 case class SEQ(r1: Rexp, r2: Rexp) extends Rexp 
@@ -12,7 +12,7 @@
 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 Left(v: Val) extends Val
@@ -22,7 +22,7 @@
    
 // some convenience for typing in regular expressions
 def charlist2rexp(s : List[Char]): Rexp = s match {
-  case Nil => EMPTY
+  case Nil => ONE
   case c::Nil => CHAR(c)
   case c::s => SEQ(CHAR(c), charlist2rexp(s))
 }
@@ -45,8 +45,8 @@
 
 // size of a regular expressions - for testing purposes 
 def size(r: Rexp) : Int = r match {
-  case NULL => 1
-  case EMPTY => 1
+  case ZERO => 1
+  case ONE => 1
   case CHAR(_) => 1
   case ALT(r1, r2) => 1 + size(r1) + size(r2)
   case SEQ(r1, r2) => 1 + size(r1) + size(r2)
@@ -58,8 +58,8 @@
 // nullable function: tests whether the regular 
 // expression can recognise the empty string
 def nullable (r: Rexp) : Boolean = r match {
-  case NULL => false
-  case EMPTY => true
+  case ZERO => false
+  case ONE => true
   case CHAR(_) => false
   case ALT(r1, r2) => nullable(r1) || nullable(r2)
   case SEQ(r1, r2) => nullable(r1) && nullable(r2)
@@ -69,9 +69,9 @@
 
 // derivative of a regular expression w.r.t. a character
 def der (c: Char, r: Rexp) : Rexp = r match {
-  case NULL => NULL
-  case EMPTY => NULL
-  case CHAR(d) => if (c == d) EMPTY else NULL
+  case ZERO => ZERO
+  case ONE => ZERO
+  case CHAR(d) => if (c == d) ONE else ZERO
   case ALT(r1, r2) => ALT(der(c, r1), der(c, r2))
   case SEQ(r1, r2) => 
     if (nullable(r1)) ALT(SEQ(der(c, r1), r2), der(c, r2))
@@ -88,7 +88,7 @@
 
 // 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)
@@ -99,7 +99,7 @@
 
 // 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)
@@ -108,19 +108,6 @@
   case Rec(x, v) => (x, flatten(v))::env(v)
 }
 
-def mkeps_all(r: Rexp) : Set[Val] = r match {
-  case EMPTY => Set(Void)
-  case ALT(r1, r2) => (nullable(r1), nullable(r2)) match {
-    case (true, true) => mkeps_all(r1).map(Left) ++ mkeps_all(r2).map(Right)
-    case (true, false) => mkeps_all(r1).map(Left)
-    case (false, true) => mkeps_all(r2).map(Right)
-  }
-  case SEQ(r1, r2) => for (v1 <- mkeps_all(r1);
-                           v2 <- mkeps_all(r2)) yield Sequ(v1, v2)
-  case STAR(r) => Set(Stars(Nil), Stars(List(mkeps(r))))
-  case RECD(x, r) => for (v <-  mkeps_all(r)) yield Rec(x, v)
-}
-
 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)
@@ -128,12 +115,10 @@
   case (SEQ(r1, r2), Right(v2)) => Sequ(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))
 }
 
-def inj_all(r: Rexp, c: Char, vs: Set[Val]) : Set[Val] =
-  for (v <- vs) yield inj(r, c, v)
 
 // main lexing function (produces a value)
 def lex(r: Rexp, s: List[Char]) : Val = s match {