--- a/progs/re1.scala	Tue Sep 20 12:25:09 2016 +0100
+++ b/progs/re1.scala	Tue Sep 20 12:38:26 2016 +0100
@@ -1,6 +1,6 @@
 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 
@@ -9,8 +9,8 @@
 // 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)
@@ -19,9 +19,9 @@
 
 // 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))
@@ -38,18 +38,18 @@
 // main matcher function
 def matches(r: Rexp, s: String) : Boolean = nullable(ders(s.toList, r))
 
-//example from the homework
+//examples from the homework
 val r = STAR(ALT(SEQ(CHAR('a'), CHAR('b')), CHAR('b')))
 der('a', r)
 der('b', r)
 der('c', r)
 
 //optional: one or zero times
-def OPT(r: Rexp) = ALT(r, EMPTY)
+def OPT(r: Rexp) = ALT(r, ONE)
 
 //n-times
 def NTIMES(r: Rexp, n: Int) : Rexp = n match {
-  case 0 => EMPTY
+  case 0 => ONE
   case 1 => r
   case n => SEQ(r, NTIMES(r, n - 1))
 }