checkpoint

[sbp.git] / src / edu / berkeley / sbp / Element.java

diff --git a/src/edu/berkeley/sbp/Element.java b/src/edu/berkeley/sbp/Element.java
index 33bfd8d..0d7fb84 100644 (file)
--- 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"); }
 
 }