X-Git-Url: http://git.megacz.com/?p=sbp.git;a=blobdiff_plain;f=src%2Fedu%2Fberkeley%2Fsbp%2FUnion.java;h=a7a76302455b8e9c051224abd3fe022a2a828bb8;hp=7274dae01ff36d1036dcaebbc99cf471e80fa315;hb=87f214f3da9f43c3ab93923313845c372f9a96be;hpb=2690f6c749f117a103c57d651d89f1b4fbf1316d

diff --git a/src/edu/berkeley/sbp/Union.java b/src/edu/berkeley/sbp/Union.java
index 7274dae..a7a7630 100644
--- a/src/edu/berkeley/sbp/Union.java
+++ b/src/edu/berkeley/sbp/Union.java
@@ -19,10 +19,11 @@ public class Union extends Element implements Iterable<Sequence> {
     void reachable(HashSet<Sequence.Position> h) { for(Sequence s : alternatives) s.reachable(h); }
 
     Topology toAtom() {
-        if (alternatives.size()==0) return null;
+        if (alternatives.size()==0) throw new RuntimeException("cannot build an Atom from a Union with no productions");
         Topology ret = null;
         for(Sequence s : this) {
             Topology a = s.toAtom();
+            if (a==null) continue;
             if (ret==null) ret = a.dup();
             else           ret.add(a.dup());
         }