--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/progs/app52.scala	Tue Aug 23 12:26:13 2016 +0200
@@ -0,0 +1,9 @@
+def ders2(s: List[Char], r: Rexp) : Rexp = (s, r) match {
+  case (Nil, r) => r
+  case (s, ZERO) => ZERO
+  case (s, ONE) => if (s == Nil) ONE else ZERO
+  case (s, CHAR(c)) => if (s == List(c)) ONE else 
+                       if (s == Nil) CHAR(c) else ZERO
+  case (s, ALT(r1, r2)) => ALT(ders2(s, r2), ders2(s, r2))
+  case (c::s, r) => ders2(s, der(c, r).simp)
+}