X-Git-Url: http://git.megacz.com/?p=sbp.git;a=blobdiff_plain;f=src%2Fedu%2Fberkeley%2Fsbp%2FElement.java;h=16ae2f04e1a874dac968f0c2b65e375cab537387;hp=33bfd8d7d08f5027e136aef1eaff959dfbf92ce7;hb=111166986ad83b54d0cae5c03c2304d23e332f29;hpb=5d18f5606c9296e6b0c5749f05fc68f358ace2f6
diff --git a/src/edu/berkeley/sbp/Element.java b/src/edu/berkeley/sbp/Element.java
index 33bfd8d..16ae2f0 100644
--- a/src/edu/berkeley/sbp/Element.java
+++ b/src/edu/berkeley/sbp/Element.java
@@ -7,12 +7,15 @@ import java.util.*;
import java.lang.reflect.*;
import java.lang.ref.*;
-/** the root superclass for all components of the grammar (terminals, nonterminals, literals, etc) */
-public abstract class Element {
+/** the root superclass for all components of the grammar (terminals, nonterminals, literals, etc) */
+public abstract class Element implements SequenceOrElement {
+
+ /** 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 null */
- 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"); }
}