diff -r 72f54acfb976 -r 22e34f93cd5d exps/bit-test.scala
--- a/exps/bit-test.scala	Fri May 10 17:23:28 2019 +0100
+++ b/exps/bit-test.scala	Tue May 14 21:43:11 2019 +0100
@@ -419,7 +419,7 @@
   (result, (end - start))
 }
 
-//size: of a Aregx for testing purposes 
+//size: of a Rrexp and ARexp for testing purposes 
 def size(r: Rexp) : Int = r match {
   case ZERO => 1
   case ONE => 1
@@ -690,6 +690,30 @@
 enum(2, "abc").map(tests_ders_bsimp(strs(1, "abc"))).
   flatten.toSet.flatten.minBy(a => asize(a._1))
 
+// tests about good
+def good (a: ARexp) : Boolean = a match {
+  case AZERO => false
+  case AONE(_) => true
+  case APRED(_, _, _) => true
+  case AALTS(_, Nil) => false
+  case AALTS(_, rs) => rs.forall(good(_))
+  case ASEQ(_, r1, r2) => good(r1) & good(r2)
+  case ASTAR(_, _) => true
+}
+
+def tests_good(r: Rexp) = {
+  val a = bsimp(internalise(r))
+  if  (! good(a) & a != AZERO) {
+        println(s"Counter-example on ${astring(a)}")
+        Some(a)  
+  } else None 
+}
+
+enum(2, "abc").map(tests_good).toSet
+
+val g1 = AALTS(Nil, Nil)
+good(g1)
+good(bsimp(g1))
 
 
 //tests retrieve and lexing