diff -r 76a98ed71a2a -r 6589afc6789b progs/re1.scala
--- a/progs/re1.scala	Mon Jan 01 15:02:03 2018 +0000
+++ b/progs/re1.scala	Thu Feb 01 21:06:20 2018 +0000
@@ -50,6 +50,12 @@
 der('b', r)
 der('c', r)
 
+val r = SEQ(SEQ(CHAR('x'), CHAR('y')), CHAR('z'))
+der('x', r)
+der('y', der('x', r))
+der('z', der('y', der('x', r)))
+
+
 //optional regular expression (one or zero times)
 def OPT(r: Rexp) = ALT(r, ONE)