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..896540e 100644 (file)
--- 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"); }
 
 }