--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/app6.scala	Thu Sep 27 11:59:41 2012 +0100
@@ -0,0 +1,11 @@
+def deriv (r: Rexp, c: Char) : Rexp = r match {
+  case NULL => NULL
+  case EMPTY => NULL
+  case CHAR(d) => if (c == d) EMPTY else NULL
+  case ALT(r1, r2) => ALT(deriv(r1, c), deriv(r2, c))
+  case SEQ(r1, r2) => 
+    if (nullable(r1)) ALT(SEQ(deriv(r1, c), r2), deriv(r2, c))
+    else SEQ(deriv(r1, c), r2)
+  case STAR(r) => SEQ(deriv(r, c), STAR(r))
+}
+