diff -r 890188804fb4 -r fb5e1ef58049 progs/comb1.scala
--- a/progs/comb1.scala	Mon Oct 24 11:58:47 2016 +0100
+++ b/progs/comb1.scala	Mon Oct 24 14:36:57 2016 +0100
@@ -34,7 +34,7 @@
 // atomic parsers  
 case class CharParser(c: Char) extends Parser[String, Char] {
   def parse(sb: String) = 
-    if (sb.head == c) Set((c, sb.tail)) else Set()
+    if (sb != "" && sb.head == c) Set((c, sb.tail)) else Set()
 }
 
 case class StringParser(s: String) extends Parser[String, String] {
@@ -55,7 +55,8 @@
 }
 
 // convenience
-implicit def string2parser(s : String) = StringParser(s)
+implicit def string2parser(s: String) = StringParser(s)
+implicit def char2parser(c: Char) = CharParser(c)
 
 implicit def ParserOps[I<% Seq[_], T](p: Parser[I, T]) = new {
   def || (q : => Parser[I, T]) = new AltParser[I, T](p, q)
@@ -78,7 +79,7 @@
   (("a" ~ Pal ~ "a") ==> { case ((x, y), z) => x + y + z } ||
    ("b" ~ Pal ~ "b") ==> { case ((x, y), z) => x + y + z } || "")
 
-println("Palindrom: " + Pal.parse_all("ababbaba"))
+println("Palindrome: " + Pal.parse_all("ababbaba"))
 
 // well-nested parenthesis parser
 lazy val P : Parser[String, String] = 
@@ -104,6 +105,20 @@
 println(E.parse_all("1+2+3"))  // this is not parsed, because of 
                                // how the grammar is set up
 
+// a repetition parser
+
+def RepParser[I  <% Seq[_], T](p: => Parser[I, T]): Parser[I, List[T]] = {
+  p ==> { case x => x :: Nil } ||
+  p ~ RepParser(p) ==> { case (x, y) => x :: y }   
+}
+
+
+// a repetition parser
+lazy val R : Parser[String, List[Char]] = RepParser('a') 
+println(R.parse_all("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"))
+
+
+
 // non-ambiguous vs ambiguous grammars
 lazy val S : Parser[String, String] =
   ("1" ~ S ~ S) ==> { case ((x, y), z) => x + y + z } || ""