X-Git-Url: http://git.megacz.com/?p=sbp.git;a=blobdiff_plain;f=src%2Fedu%2Fberkeley%2Fsbp%2FSequence.java;h=d8f39fd860301538c3eee496144f19564f9ad6e6;hp=d274013904da23504c899ae9ab62651a2f7e1c63;hb=aeae43c3255b14e4ba65e0171b87c48bbf953c31;hpb=cd979dfc5968a185c12d6eedc2aa688ef4b0f8b8

diff --git a/src/edu/berkeley/sbp/Sequence.java b/src/edu/berkeley/sbp/Sequence.java
index d274013..d8f39fd 100644
--- a/src/edu/berkeley/sbp/Sequence.java
+++ b/src/edu/berkeley/sbp/Sequence.java
@@ -89,6 +89,14 @@ public abstract class Sequence extends Element implements Iterable<Element> {
     /** the imaginary position before or after an element of a sequence; corresponds to an "LR item" */
     public class Position {
 
+        private Forest zero = null;
+        public Forest zero() {
+            if (zero != null) return zero;
+            if (pos > 0) throw new Error();
+            return zero = rewrite(null);
+        }
+
+
                 final int      pos;
         private final Position next;
                 final Forest[] holder;
@@ -113,7 +121,7 @@ public abstract class Sequence extends Element implements Iterable<Element> {
         /** true iff this Position is the last one in the sequence */
         public boolean isLast() { return next()==null; }
 
-        // Reduction /////////////////////////////////////////////////////////////////////////////////
+        // Position /////////////////////////////////////////////////////////////////////////////////
 
         final <T> Forest<T> rewrite(Token.Location loc) {
             if (this==firstp()) return epsilonForm();