X-Git-Url: http://git.megacz.com/?p=sbp.git;a=blobdiff_plain;f=src%2Fedu%2Fberkeley%2Fsbp%2FElement.java;fp=src%2Fedu%2Fberkeley%2Fsbp%2FElement.java;h=0d7fb84f583ea0ba9594a9b23cf88d83998afafc;hp=33bfd8d7d08f5027e136aef1eaff959dfbf92ce7;hb=6b24a8ff6410a47d7290c2b42587fd8a8c5341e8;hpb=5d18f5606c9296e6b0c5749f05fc68f358ace2f6

diff --git a/src/edu/berkeley/sbp/Element.java b/src/edu/berkeley/sbp/Element.java
index 33bfd8d..0d7fb84 100644
--- a/src/edu/berkeley/sbp/Element.java
+++ b/src/edu/berkeley/sbp/Element.java
@@ -12,7 +12,7 @@ public abstract class Element {
 
     abstract StringBuffer toString(StringBuffer sb);
 
-    /** if this element always matches exactly one token, return a topology covering exactly those possible tokens, otherwise <tt>null</tt> */
-    Forest epsilonForm() { return null; }
+    /** returns the Forest resulting from matching this element against the empty string */
+    Forest epsilonForm() { throw new Error("element " + this + " has no epsilon form"); }
 
 }