Add a _ROOT_ description which matches exactly the root of a tree

Author: AngledLuffa

src/edu/stanford/nlp/trees/tregex/DescriptionPattern.java

@@ -18,7 +18,7 @@ public class DescriptionPattern extends TregexPattern  {
   private static Redwood.RedwoodChannels log = Redwood.channels(DescriptionPattern.class);
 
   enum DescriptionMode {
-    PATTERN, STRINGS, EXACT, ANYTHING
+    PATTERN, STRINGS, EXACT, ANYTHING, ROOT
   }
 
   private final Relation rel;
@@ -77,6 +77,11 @@ public DescriptionPattern(Relation rel, boolean negDesc, String desc,
         descPattern = null;
         exactMatch = null;
         stringFilter = null;
+      } else if (desc.equals("_ROOT_")) {
+        descriptionMode = DescriptionMode.ROOT;
+        descPattern = null;
+        exactMatch = null;
+        stringFilter = null;
       } else if (SINGLE_WORD_PATTERN.matcher(desc).matches()) {
         // Expressions are written like this to put special characters
         // in the tregex matcher, but a regular expression is less
@@ -403,6 +408,9 @@ private void goToNextTreeNodeMatch() {
             case STRINGS:
               found = myNode.stringFilter.test(value);
               break;
+            case ROOT:
+              found = (nextTreeNodeMatchCandidate == root);
+              break;
             default:
               throw new IllegalArgumentException("Unexpected match mode");
             }