X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=src%2Fedu%2Fberkeley%2Fsbp%2FElement.java;h=896540e904f0868bfe1dd4f68ee3fa630ecde6b9;hb=23091054a6842031d168c2628216c2310149cc5d;hp=33bfd8d7d08f5027e136aef1eaff959dfbf92ce7;hpb=5d18f5606c9296e6b0c5749f05fc68f358ace2f6;p=sbp.git

diff --git a/src/edu/berkeley/sbp/Element.java b/src/edu/berkeley/sbp/Element.java
index 33bfd8d..896540e 100644
--- a/src/edu/berkeley/sbp/Element.java
+++ b/src/edu/berkeley/sbp/Element.java
@@ -10,9 +10,12 @@ import java.lang.ref.*;
 /** the root superclass for all components of the grammar (terminals, nonterminals, literals, etc) */
 public abstract class Element {
 
+    /** sorry, you can't make up new, custom elements */
+    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"); }
 
 }