Backreferencing works fine for relation names. Update the documentation and add a test of the feature

Author: AngledLuffa

src/edu/stanford/nlp/semgraph/semgrex/SemgrexPattern.java

@@ -169,10 +169,10 @@
  *
  * It is also possible to name relations.  For example, you can write the pattern
  * {@code {idx:1} >=reln {idx:2}}  The name of the relation will then
- * be stored in the matcher and can be extracted with {@code getRelnName("reln")}
- * At present, though, there is no backreferencing capability such as with the
- * named nodes; this is only useful when using the API to extract the name of the
- * relation used when making the match.
+ * be stored in the matcher and can be extracted with {@code getRelnName("reln")}.
+ * If the relation is later referenced a second time, the type of
+ * relation must be the same, or the potential match will not be
+ * accepted.
  * <p>
  * In the case of ancestor and descendant relations, the <b>last</b>
  * relation in the sequence of relations is the name used.

test/src/edu/stanford/nlp/semgraph/semgrex/SemgrexTest.java

@@ -718,6 +718,56 @@ public void testNamedRelation() {
     assertFalse(matcher.find());
   }
 
+  /**
+   * The named relation feature should incorporate backreferences
+   */
+  public void testNamedRelationBackreference() {
+    SemanticGraph graph = SemanticGraph.valueOf("[ate subj>Bill obj>[muffins compound>blueberry]]");
+
+    SemgrexPattern pattern = SemgrexPattern.compile("{}=A >=foo ({}=B >=foo {}=C)");
+    SemgrexMatcher matcher = pattern.matcher(graph);
+    assertFalse(matcher.find());
+
+    graph = SemanticGraph.valueOf("[ate dep> [Bill dep> cat]]");
+    matcher = pattern.matcher(graph);
+    assertTrue(matcher.find());
+    assertEquals("ate", matcher.getNode("A").toString());
+    assertEquals("Bill", matcher.getNode("B").toString());
+    assertEquals("cat", matcher.getNode("C").toString());
+    assertEquals("dep", matcher.getRelnString("foo"));
+    assertFalse(matcher.find());
+
+    graph = SemanticGraph.valueOf("[ate cop> [Bill dep> cat]]");
+    matcher = pattern.matcher(graph);
+    assertFalse(matcher.find());
+
+    graph = SemanticGraph.valueOf("[ate dep> [Bill cop> cat]]");
+    matcher = pattern.matcher(graph);
+    assertFalse(matcher.find());
+
+    graph = SemanticGraph.valueOf("[antennae amod> big amod> blue]");
+    pattern = SemgrexPattern.compile("{}=A >=foo {}=B >=foo ({}=C !== {}=B)");
+    matcher = pattern.matcher(graph);
+    assertTrue(matcher.find());
+    assertEquals("antennae", matcher.getNode("A").toString());
+    assertEquals("big", matcher.getNode("B").toString());
+    assertEquals("blue", matcher.getNode("C").toString());
+    assertEquals("amod", matcher.getRelnString("foo"));
+
+    assertTrue(matcher.find());
+    assertEquals("antennae", matcher.getNode("A").toString());
+    assertEquals("blue", matcher.getNode("B").toString());
+    assertEquals("big", matcher.getNode("C").toString());
+    assertEquals("amod", matcher.getRelnString("foo"));
+
+    assertFalse(matcher.find());
+
+    graph = SemanticGraph.valueOf("[antennae amod> big dep> blue]");
+    pattern = SemgrexPattern.compile("{}=A >=foo {}=B >=foo ({}=C !== {}=B)");
+    matcher = pattern.matcher(graph);
+    assertFalse(matcher.find());
+  }
+
   public void testAttributeConjunction() {
     // A possible user submitted error: https://github.com/stanfordnlp/CoreNLP/issues/552
     // A match with both POS and word labeled should have both attributes on the same node