--- a/progs/re0.scala	Sun May 07 03:01:29 2017 +0100
+++ b/progs/re0.scala	Tue May 09 12:31:55 2017 +0100
@@ -1,4 +1,5 @@
 import scala.annotation.tailrec
+import scala.language.implicitConversions
 
 abstract class Rexp
 
@@ -72,6 +73,7 @@
     if (i == 0) NULL else SEQ(der(c, r), REP(r, i - 1))
 }
 
+
 // derivative w.r.t. a string (iterates der)
 @tailrec
 def ders (s: List[Char], r: Rexp) : Rexp = s match {